The iBooks app for iOS lets you assign your books to different collections, but does not have any obvious way for you to see all of your books, regardless of the collection you have put them in. There is, however, a workaround that can show you just about all of your books at once: reveal the search field at the top of any collection in iBooks and type a single space into that field.
With this search, iBooks lists all of the books that have a space either in the title of the book or in the author's name. Other than the rare book that has a one-word title and a single-name author, you end up with a list of all of your books.
From Michael E. Cohen