The only slang dictionary I've found that includes an entry for "pig book" is Tom Dalzell & Terry Victor, The New Partridge Dictionary of Lang and Unconventional English (2006), and its coverage is pretty skimpy:
pig book noun a student directory with photographs of each student | US, 1969 | —Current Slang p. 17, Spring 1971
Searches of the Google Books and Elephind book and newspaper databases yield the following instances of pig book from the 1960s, 1970s, and 1980s, the earliest one being from 1964. I have grouped these instances by institution and chronologically, so the universities appear in the order in which published reference is first made to the term pig book in connection with them.
Cornell University (Ithaca, New York)
The earliest match for this school is from 1964 and is a discovery of ace researcher and English Language & Usage contributor DjinTonic. From what appears to be a review of the social scene at Cornell during the 1963–1964 academic year, in The Cornellian (1964), Cornell's yearbook [combined snippets]:
The human touch comes largely from miscellaneous social fragments. There was the reviled and reviling "pig book" of our class, with a certain scandalous picture of a torso. [Harry] Belafonte got laryngitis and [Johnny] Mathis finked out; and big weekends continued their decline. But we heard Ray Charles, the Limelighters, the Weavers, Odetta, Joan Baez, Carlos Montoya, Louis Armstrong, Dave Brubeck, as well as [Eugene] Ormandy, [George] Szell, [David] Oistrakh, and [Isaac] Stern.
Also, from "The 1968 Cornell Summer Work-scholarship Program" (1968):
He looked me up in the Pigbook, and told me he had a daughter working in Mann State Hospital.
He asked numerous questions, trying to feel me out. He wouldn't look into my face. He asked about my family, where I worked previously, ...
From an advertisement for Alpha Phi Omega Fraternity in the [Ithaca, New York] Cornell Daily Sun (February 26, 1971):
Cornell men and women—serve the university community through APO ... '75 Pig Book
From The Cornellian (1983) [combined snippets]:
The Gamma Chapter of Alpha Phi Omega was established at Cornell in 1927 based on the Scouting principles of leadership, friendship, and service to the Cornell campus and the Ithaca community. One of our most important campus activities is Cornell Campus Chest, a fund-raising drive, which includes activities such as Ugly Man on Campus and Miles of Pennies. Proceeds from the drive go to several campus and community organizations. Other campus services include publication of the Freshman Register (Pigbook), coordination of the annual Blood Drive, and distribution of soda and candy at cafeteria.
And from Trevor Connor, Get a Clue: The "Real" Guide to Cornell and Ithaca 1993–1994 (1994), in a glossary of terms used at Cornell:
Pigbook—the Freshman directory
Case Western Reserve University (Cleveland, Ohio)
From 'Pig Book' Gets Subsidy," in the [Cleveland, Ohio] Reserve Tribune (March 3, 1966), the student newspaper of Case Western Reserve University :
'Pig Book' gets subsidy
Freshman Class Directory, traditionally published through the joint efforts of the Inter-Fraternity Council and Mather Gov, will additionally subsidized this year by the Adelbert Student Council.
From a classified ad published in the *[Cleveland, Ohio] Observer(November 7, 1969), also a student newspaper of Case Western Reserve University:
WANTED: Copy of sophomore pig book. Will pay up to $1 for it. Ideal opportunity for students intending to transfer or other with no special attachments to such items. Contact Bill, 220 Cutler House, ext. 3160.
Sixteen additional references to "the Pig Book" appear in issues of the Observer and its successor the Case Tech in 1971, 1972, 1973, and 1974.
American University (Washington, D.C.)
From a letter to the editor of the [Washington D.C.] American Eagle (October 2, 1970), the student newspaper of American University:
The Pig Book
To the editor"
Congratulations! I have just seen a copy of the 1969 New Student Photo Directory and found it two months late, of poor quality and above all abounding with errors. I find it hard to believe that you even took the time to print it at all its usefulness is now almost nonexistent.
From "Americana" in the same newspaper (October 2, 1970):
Handbook (Pigbook) Anyone desiring this year's student handbook may pick one up in Room 200 MGC.
Ans a week later (October 9, 1970) in the same column in the same newspaper:
Frosh Pigbook Due to a fire at the printing plant, "The Freshmen Pictorial Register," alias "The pigbook" will be delayed two or three weeks. Mike Passet of the RHA hopes to have them distributed individually in the dorms.
Franklin and Marshall College (Lancaster, Pennsylvania)
From "Pie Throwers Accept Contracts on Students," in the [Lancaster, Pennsylvania] F&M College Reporter (March 1, 1977), the student newspaper at Franklin and Marshall College:
As I understand now, for a mere $3.50, one of the members of the pie-throwing organization would hurl a whipped cream pie "right between the eyes" of any F and M student you desire. And to make sure you got your money's worth, the members would sacrifice many hours of their precious study time to covertly plan their method of cream-ation. I imagine they got really excited when offered a contract to knock off a pre-med or pre-law. Even a woman would not be spared, unless, of course, one of the members fell madly in love with her picture in the pig-book.
From "Public Relations Office Projects College's Image," in the [Lancaster, Pennsylvania] F&M College Reporter (February 19, 1980):
One of the most important freshman possessions, the Novice's Guide to Fumming (alias "pig book") , also originates in the "Old Main propaganda center."
Seven subsequent references to pig book appear in this newspaper in 1981 and 1982.
Gettysburg College (Gettysburg, Pennsylvania)
From "A Frosh Impression," in the [Gettysburg, Pennsylvania] Weekly Gettysburgian (September 13, 1988):
They have scribbled their names on white Tshirts. They have been divided into teams to share a balloon. Their names and faces can be found in something called a "PIG book." They are not allowed in fraternities, and they still don't know how to use the library. Yes, the freshman class of 1992 is here.
Tufts University (Medford, Massachusetts)
According to George Colt, November of the Soul: The Enigma of Suicide (2006) the expression was also used at Tufts University near Boston at some unspecified earlier date:
On a September evening not long after she had arrived at her freshman year at Tufts University in Medford Massachusetts, Merryl Maleska lay on a bed in her dorm room, flipping through the “pigbook,” a thin yellow volume containing pictures of everyone in her class.
A different (and more recent) meaning of 'pig book'
In more recent years (since 1992, by my math), "Pig Book" has been the name of an annual report on supposedly wasteful government spending in the US published by a group closely aligned with the Republican Party that calls itself Citizens Against Government Waste; the usage in this case is a play on the term pork, which, as applied to government spending, refers to funding of projects that are popular in their sponsors' home districts but are unnecessary, fiscally unsound, or otherwise dubious as a matter of policy.
A different (and older) meaning of 'pig book'
Decades earlier, an instance of pig book appears in William Crealock, Vagabonding Under Sail (1951) [combined snippets]:
We had no visitors' book of the ordinary kind on board Content; somehow we felt that it was not very appropriate. Instead we had our 'elephant book;' in this our friends had to draw an elephant while keeping their eyes closed. The results were wonderful, no doubt of deep psychological significance , and usually threw our guests into mild hysterics; so much better than a bald signature and the numbing impact of that awful column headed 'Remarks.'The idea was not our own but came from our friends on the Arthur Rogers who had their 'pig book.'
But an item titled "A Pig-Book" in the [Friday Harbor, Washington] San Juan Islander (November 9, 1907) indicates that this type of "pig book" is goes back at least to the first decade of the twentieth century:
If you would like to have a perfect treasure-house of amusement to show to your friends, get up a "pig-book." But what In the world is a pig-book? Get a book made up of forty or fifty pages of blank paper, and then ask forty or fifty different persons each to draw, with his or her eyes shut, a picture of a pig on one of the pages. The amateur artist will probably succeed pretty well until he comes to putting on the pig's tail; that appendage is sometimes drawn in the most impossible place, and the result is very amusing. You have no conception of the fun you may have out of a "pig-book" until you get one. Try it.
The earliest confirmed match that I'm aware of for pig book in the sense of "photo directory of (usually new) students at a college or university" is from 1964, in The Cornellian, the yearbook of Cornell University in Ithaca, New York. Other instances of the term pop up in the 1960s, 1970s and 1980s in Ohio, Washington DC, Pennsylvania, and Massachusetts. I didn't find any instances of the term from these three decades west of Cleveland or south of Washington DC, suggesting that the term was regional, not national, during that period (and may still be regional today).
None of the sources that I checked hazarded an opinion about why or how the directories acquired the slang name pig book. The simplest explanation might be that pig book evolved in stages, from picture book to pic book to pig book. I have no evidence that this is what happened, however.
The editors of the 1964 Cornellian yearbook characterize the photo directory of students—presumably intended to help students learn the names of others on campus—as "reviled and reviling" immediately before referring to it (with quotation marks) as "pig book." This certainly strengthens the case for interpreting pig book as a pejorative term, and not merely a jokey one, from an early date.
Although the term may have caught on in part because of intended or unintended connotations of pig as an insulting description of the people whose pictures appeared in the directory or as a comment on the sexism of some users of the directory, neither of these possible explanations of the term's origin seems overwhelmingy likely to me. One over-the-top reference to "the Pig Book" appears in Steven Crist & George Meyer, The Harvard Lampoon Big Book of College Life (1978), in which the narrator talks about using the directory as a way to identify prospective love interests:
Glancing through Freshman Directory (which in a wit fit I had dubbed the Pig Book), I spotted a girl who looked like Russ Meyer's bath toy.
But this narrator's other adventures in the same article (which I had to reconstruct piece by piece as a set of adjacent snippet views) suggest that he is a ludicrously self-aggrandizing figure and not to be taken as the suave masher he imagines himself to be.
Likewise, I found no mention of the phrase "personal identification guide" in any of the student newspapers from the 1960, 1970s, and 1980s that show up above as sources of early instances of the slang term.