We have always used free books available on line for our school. You can choose to download them and store them on the computer or just bookmark them. I usually put the current selections on my computer desktop so the children can access them easily. When we are done with the book we delete it from our files.
http://www.gutenberg.org/wiki/Main_Page ( my favorite !)
http://www.magickeys.com/books/ ( for young children )
http://www.bartleby.com/ ( we use this one for Grays Anatomy )