The cheap and fast way is to get some construction paper, white off, and popsicle sticks.
Take the sticks, and wrap them in colored construction paper on both wide sides and one of the small sides (leave the last thin edge uncovered), glue it in place with a little bit of an overlap. Let it dry completely.
Then take some heavy scissors and cut it to the length of the books you want (If you want thicker or smaller books, use different sized sticks). Then on the exposed wood, hit it with the white off/liquid paper...carefully. Let it dry and use a fine pen for whatever covers you want.
You can use something similar for openable books, just stack two sticks (Don't glue them together) and glue a piece of folded white paper inside, and glue the construction paper to the outer wide sides while leaving the thin side where you would glue for the unopenable one unglued, but covered. Cut to length and hit the exposed wood with the white off/liquid paper.
You can make a rather massive pile of books with those two options quickly. Sure, the openable aren't quite the normal looking ones, but it's fast, easy, and you can open/close them and it's not to hard to use more paper to add other stuff to make it look right.