My original notion was,

• A) If there's a movement and a destination (as in the case of thumbing a book to reach a certain page), it should be to:
  Class, open your books to page 13!

• B) If there's none, then at:
  There on the counter rested my cookbook, open at page 13.
  The tome fell open right at the middle page.

Then I asked around and rummaged in corpora and found out that my reasoning was flawed. [IMO, there's either something wrong with logic or with language—or both. Somebody fix the whole mess please.] In fact, when it comes to opening or being open, the preposition is almost always to in AmE, whereas BrE is probably mixed, and (sometimes?) there is even a strong preference for at (especially in cases like B above).

I'll close with some of the search results that I mentioned, and leave the rest to others to hash out. Speakers of other dialects are more than welcome to chip in as well!

1. shut the book for a moment, then open it back up to page one and begin again – COCA
2. Students, if you could please open your math books to page two – COCA
3. Open your grammar at page fourteen – BNC
4. every time I took up the book it opened at page 92, although I have never deliberately read that page – BNC
5. I happened to open the Rome Treaty at page 89 of the English text – Hansard Corpus

My original notion was,

• A) If there's a movement and a destination (as in the case of thumbing a book to reach a certain page), it should be to:
  Class, open your books to page 13!

• B) If there's none, then at:
  There on the counter rested my cookbook, open at page 13.
  The tome fell open right at the middle page.

Then I asked around and rummaged in corpora and found out that my reasoning was flawed. [IMO, there's either something wrong with logic or with language—or both. Somebody fix the whole mess please.] In fact, when it comes to opening or being open, the preposition is almost always to in AmE, whereas BrE is probably mixed, and (sometimes?) there is even a strong preference for at (especially in cases like B above).

I'll close with some of the search results that I mentioned, and leave the rest to others to hash out. Speakers of other dialects are more than welcome to chip in as well!

1. shut the book for a moment, then open it back up to page one and begin again – COCA
2. Students, if you could please open your math books to page two – COCA
3. Open your grammar at page fourteen – BNC
4. every time I took up the book it opened at page 92, although I have never deliberately read that page – BNC
5. I happened to open the Rome Treaty at page 89 of the English text – Hansard Corpus
6. his first act on sitting down to breakfast was to open the tabloid at page three, fold it and prop it against the sugar bowl – BNC

My original notion was,

• A) If there's a movement and a destination (as in the case of thumbing a book to reach a certain page), it should be to:
  Class, open your books to page 13!

• B) If there's none, then at:
  There on the counter rested my cookbook, open at page 13.
  The tome fell open right at the middle page.

Then I asked around and rummaged in corpora and found out that my reasoning was flawed. [IMO, there's either something wrong with logic or with language—or both. Somebody fix the whole mess please.] In fact, when it comes to opening or being open, the preposition is almost always to in AmE, whereas BrE is probably mixed, and (sometimes?) there is even a strong preference for at (especially in cases like B above).

I'll close with some of the search results that I mentioned, and leave the rest to others to hash out. Speakers of other dialects are more than welcome to chip in as well!

1. she snapped open the Daily News and immediately turned to Page Six – COCA
2. Students, if you could please open your math books to page two – COCA
3. Open your grammar at page fourteen – BNC
4. every time I took up the book it opened at page 92, although I have never deliberately read that page – BNC
5. I happened to open the Rome Treaty at page 89 of the English text – Hansard Corpus

My original notion was,

• A) If there's a movement and a destination (as in the case of thumbing a book to reach a certain page), it should be to:
  Class, open your books to page 13!

• B) If there's none, then at:
  There on the counter rested my cookbook, open at page 13.
  The tome fell open right at the middle page.

Then I asked around and rummaged in corpora and found out that my reasoning was flawed. [IMO, there's either something wrong with logic or with language—or both. Somebody fix the whole mess please.] In fact, when it comes to opening or being open, the preposition is almost always to in AmE, whereas BrE is probably mixed, and (sometimes?) there is even a strong preference for at (especially in cases like B above).

I'll close with some of the search results that I mentioned, and leave the rest to others to hash out. Speakers of other dialects are more than welcome to chip in as well!

1. shut the book for a moment, then open it back up to page one and begin again – COCA
2. Students, if you could please open your math books to page two – COCA
3. Open your grammar at page fourteen – BNC
4. every time I took up the book it opened at page 92, although I have never deliberately read that page – BNC
5. I happened to open the Rome Treaty at page 89 of the English text – Hansard Corpus

My original notion was,

• A) If there's a motion and a destination (as in the case of thumbing a book to reach a certain page), it should be to:
  Class, open your books to page 13!

• B) If there's none, then at:
  There on the counter rested my cookbook, open at page 13.
  The tome fell open right at the middle page.

Then I asked around and rummaged in corpora and found out that my reasoning was flawed. [IMO, there's either something wrong with logic or with language—or both. Somebody fix the whole mess please.] In fact, when it comes to opening or being open, the preposition is almost always to in AmE, whereas BrE is probably mixed, and (sometimes?) there is even a strong preference for at (especially in cases like B above).

I'll close with some of the search results that I mentioned, and leave the rest to others to hash out. Speakers of other dialects are more than welcome to chip in as well!

1. she snapped open the Daily News and immediately turned to Page Six – COCA
2. Students, if you could please open your math books to page two – COCA
3. Open your grammar at page fourteen – BNC
4. every time I took up the book it opened at page 92, although I have never deliberately read that page – BNC
5. I happened to open the Rome Treaty at page 89 of the English text – Hansard Corpus

My original notion was,

• A) If there's a movement and a destination (as in the case of thumbing a book to reach a certain page), it should be to:
  Class, open your books to page 13!

• B) If there's none, then at:
  There on the counter rested my cookbook, open at page 13.
  The tome fell open right at the middle page.

Then I asked around and rummaged in corpora and found out that my reasoning was flawed. [IMO, there's either something wrong with logic or with language—or both. Somebody fix the whole mess please.] In fact, when it comes to opening or being open, the preposition is almost always to in AmE, whereas BrE is probably mixed, and (sometimes?) there is even a strong preference for at (especially in cases like B above).

I'll close with some of the search results that I mentioned, and leave the rest to others to hash out. Speakers of other dialects are more than welcome to chip in as well!

1. she snapped open the Daily News and immediately turned to Page Six – COCA
2. Students, if you could please open your math books to page two – COCA
3. Open your grammar at page fourteen – BNC
4. every time I took up the book it opened at page 92, although I have never deliberately read that page – BNC
5. I happened to open the Rome Treaty at page 89 of the English text – Hansard Corpus