it sort of depends how you got books on your device in the first place.
get a file manager, open up the sd card, and you will probably see a folder called "books", or maybe "ebooks", or something like that. just delete it from your file manager.
some ereader apps may also store books in their own data folders. again, a file manager can help you navigate to it.
if kindle app, i have no idea.
if google play books, you can delete from the google play books app.