They are forms of middle-voice, in that they are neither the active or the passive voice. To reiterate those voices first before examining the middle, the active voice is where the subject is the agent or actor; the thing that does the action:
The bird flew.
The boy kicked the ball.
The boy is kicking the ball.
The passive voice is where the subject is the patient; the thing the action was done to:
The man was shot.
The ball was kicked by the boy.
The ball is being kicked by the boy.
It's worth noting at this point that while the second and third example of each group convey the equivalent information, the first example of each cannot be stated in the other voice. The first active example uses flew in an intransitive sense, so it can't be made passive (though it can if used with a preposition; "The bird flew at him"/"He was flown at by the bird"). The first passive example can't use the active because the agent isn't stated, so while we can say something like "Somebody shot the man" we're having to add this "somebody" and can't just leave it out.
We'll also note that the third example of each is progressive, because this will come up later.
Some constructions have features of both of these voices, and so are called "middle voice". In the contexts of other languages some middle-voice constructions are more passive than others and so some get called mediopassive (that is, in between middle and passive) but in terms of English you will often get middle and mediopassive used of the same phrases:
The dinner is cooking.
The document is printing.
Bread baked in the oven.
Here the form is pretty much the same as the active form, but the subject is the patient as in the passive.
Or is it? The sense here is that the subject is "doing" something. It is the document that is doing whatever the verb "print" might mean, even though by the definitions we know of these verbs they can only really be understood as the patient (really, the document is being printed). It's somewhere in-between the active and passive voice; it's a middle voice.
There are a few interesting things about this middle voice, not least that we tend not to hear about it. Native speakers generally don't learn about it in school even if they do learn about active and passive voices and ESL classes often skip it too. This is one of the first things that will lead some people to claim it is wrong; they'd have no qualms about saying "the dinner is cooking" but when they come to examine a clause in the middle voice they find it doesn't fit into the categories they know of and rather than suppose there must be at least one other category, they reject the clause.
The even more interesting thing about it, is that even those people who fully accept many middle voice constructs will still not use it a lot of the time. Almost nobody would for example say "the house is building" or "the trunks were carrying down", but rather they would say "the house is being built" or "the trunks were being carried down". Somehow bread baking is okay but houses building is not.
And I picked those two examples for a reason, as they are examples I can find from earlier in the language's history:
The clock struck ten while the trunks were carrying down, and the general had fixed to be out of Milsom Street by that hour.
That's a quote from Northanger Abbey by Jane Austen. It's a well-regarded novel, but here we have precisely the "trunks were carrying down" that I just said English speakers wouldn't say.
The significant difference is, that this is 2015 and that was 1803. At the time, constructions like this were in no way controversial.*
What would have been controversial then is "the trunks were being carried down".
This progressive passive use was relatively novel at the time, having started in the latter half of the previous century, and at the time was frowned upon by traditionalists†:
This modern form is very seldom used among writers of the highest class. The best writers say “The house is building” not “The house is being built.” “An act not less horrid was perpetrating in Eskdale.”—Macaulay. “Chelsea hospital was building.”—Id. — Nobel Butler, A Practical and Critical Grammar of the English Language, 1874
There were many other such complaints‡, but of course people also just differ in style and Austen probably wasn't being a conscious traditionalist to judge by her use of other features some frowned upon but she never used the progressive passive while the Romantic Poets were very fond of it. Dickens used both alternatives.
So what we have here is a time in the history of the language when someone who wanted to express an ongoing action in terms of the patient had a choice: They could use the middle-voice progressive (the passival)§ or the passive-voice progressive.
Since the passive-voice progressive is no longer controversial** I don't need to tell you that it's now more often used. Because it grew so much we are now at the point where if we want to express an ongoing action in terms of the patient we will almost always use it instead of the passival, and with one of the tasks the middle-voice served no longer served by it, it has massively declined. Non-progressive uses of the middle voice ("the bread baked") are also relatively rare.
So why do we still find it sometimes?
One case is reflexively, that is where the agent is also the patient. "Bob shaves" is immediately understood as meaning "Bob shaves himself".
Some survivals can be considered set-phrases; "the dinner was cooking" and "the bread was baking" (along with "the dinner cooked" and "the bread baked") were likely so common that people just kept the form and thought it active (that cook had a sense in which it is something food did), especially since most of them would not have regularly remarked upon the fact that they were slowly and collectively moving from preferring the passival to preferring the passive progressive over the course of many decades.
But still, while there are many cases where the passival and progressive could both have been used to express the same thing, there are times when one doesn't serve.
"The wolf is hunting" would only ever have been understood as meaning that the wolf was looking for prey, and the passival cannot therefore express what the passive progressive can with "the wolf is being hunted".††
Conversely, "the car handles smoothly" requires that part-way point between active and passive to make it about the car. "The car is being handled smoothly" suggests that it is the driver who is responsible for this smoothness while "the car handles smoothly" suggests it is due to a quality of the car. What would we do without the middle voice? "The handling of the car is smooth" literally expresses the same thing, but in conjuring a new subject ("the handling of the car") we've moved away from the singular concept of the car and so lost some nuance. Other alternatives have similar failings. Certainly there is no other way to convey the same thing with just the noun phrase "the car" the verb "handle" and the adverb "smoothly".
Likewise "the document is printing" cannot be recast into the active, because doing so requires either "someone is printing the document" or "the printer is printing the document", and really it means neither and both and that's part of the point! While the passive ("the document is being printed") doesn't state the agent it leaves room for one ("…printed by…") in a way that "the document is printing" does not. We don't merely want to leave the agent out, we want to move the document part-way to being the agent, to have something in-between the active and the passive, to have a middle voice.
So too with your examples. "This book reads easily" describes a quality of the book in terms of the book, the verb read and the adjective easily in a way no other form can. "The book is an easy read" serves better here than "the handling of the car is smooth" above, but it's still not quite the same thing.
It is close enough that a lot of people would still say "it is an easy read" rather than "it reads easily", but it is also far enough that some people would not.
Likewise "this book is reading easily" describes an ongoing experience of the book in terms of the book and so while it means the same thing as "I am finding that reading this book is easy" this loses a sense of this being a quality of the book rather than of the reader.
And so, while the middle voice is not used anywhere near as much now as it was two hundred years ago, there are still times when it serves much better.
And as a rule, it is only those times that we use it. If something can be expressed just as well and as succinctly in the active or passive, we will generally use one of those. We would not expect "the house is building" or "the trunks were carrying down" any more because "the house is being built" and "the trunks were being carried down" serve just as well. Where people draw the line will vary from person to person. One should always avoid it if it can cause ambiguity, as in "the wolf is hunting" above
*It would have been a couple of hundred years earlier again, when some people found the change from "the trunks were a-carrying" to "the trunks were carrying" objectionable. Not only is the language always changing, but it's always being complained about.
†The main objections at the time were two-fold:
The "being built" of "the house is being built" clashing with the sense it has in "being built of straw, the pig's house blew down easily".
The fact that "is being", "are being" etc. uses two cases of the verb to be consecutively, which sounded clumsy to many 18th and 19th century ears much as other consecutively repeated words can still sound clumsy today. It struck many then as starkly illogical.
‡Richard Grant White's attack on the passive progressive, "Is Being Done" is an outstanding piece of ranting and anyone who finds dark humour in observing a flame-war will find it a rewarding read for that reason alone.
§Many people at the time would not have understood the passival as "middle-voice" even when it was in much more common use. Most would have understood it as passive, though a defence of the passive progressive in The New York Teacher, and the American Educational Monthly, Volume 6 of 1869 argues that the passival is in fact active but works figuratively to produce "this seemingly ‘passive sense’".
**There are people who have a different sort of complaint against the passive voice generally, but that's not relevant here, and such people can rarely identify the passive voice anyway so whether you use it or not has no bearing on whether they'll complain about your using it.
††But consider, "Take it easy. I don't slap so good around this time of evening." from the film The Big Sleep (1946). Here because we've just seen Lauren Bacall slap Humphrey Bogart, we've extra-lingual information about who is the agent and who is the patient of slap, and so it works reasonably well. (Only "reasonably well" as Bogart's character is still deliberately a bit strange in phrasing to express sarcasm in his very suggestion that he might "slap better" at another time of the day.).