Books are normally divided in to separate pages of text that are observed word by word. The reader flicks from one page to the next. The design of a book front cover expresses what the book is about. The actual typography within books contains very little expression, rightly so, all designed for the clarity of reading.
Book posters were designed to release all of the text into a whole image of expression. Freeing it from the limitations of the book to create a new typographical expression to the words and language.
Book posters I have to confess is a gimmick, a game. Viewed from a distance like a book cover and then as then differently as a viewer moves closer. The observer then can see that the overall image is all made up of words and contains the whole book is one visual bang.
Such a quick visual bang is also an opportunity to work with the typography to express the books theme, emotion, or idea. Allowing the reader to understand the book without reading a single word.