Books are normally divided in to separate pages of text that are observed word by word as a reader flicks from one page to the next. The front cover of most books attempt to express what the book is about, which can be a very difficult task almost impossible for some books. The actual typography within the 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 the viewer moves closer they then see that the form of the image is all made up of words and contains the whole book is one visual bang.