As I’ve mentioned before, bamboo drawer organizers make wonderful bookcases with little to no modification.
Make page blocks for dummy books by hot-gluing together "signatures" of balsa, foamcore or craft foam cut to size.
Don’t forget to vary the dimensions of the dummy books. No one’s library contains uniform volumes of all the same size.
Use scraps of paper sticking out from between "signatures" for bookmarks.
Create the impression of a diverse library by binding your dummy books in a variety of papers. Magazine pages and origami paper provide aesthetically pleasing patterns. Used cheap self-adhesive gift tags for trim, stripes and further differentiation.
While you’re at it, don’t forget to distribute colors and patterns randomly amongt the stacks. Very few people shelve according to the color spectrum. :p
For added realism, make it messy. [Tip for the lazy: Falling-over books take up more room than neatly stuffed shelves!] Have some books stick out further than others. A pile of books can act as a makeshift bookend. Try shelving some books on top of other books.
Use some sort of adhesive to keep the books on the shelves, particularly those that are falling over or leaning precariously.
See these principles in practice below.