Are you doing an open book cake or a closed book cake?
If you do an open book cake, you can definately do it in buttercream. I'm not sure about a closed book if you want to the cover to have a lip. If you want it to have a lip, then you would probably want to do it in fondant. If you're looking for some real detailed help Sharon Z has a DVD on doing a book cake and it's awesome. http://www.sugaredproductions.com/product/Book-Smarts-DVD/
I used her DVD to learn how to do a book for my competition piece http://cakecentral.com/gallery/2021758