This is something that comes up quite a lot, as people are always looking for images that they can use without fear of payment or copyright restrictions. There's a nice new collection of such engines over at the Hootsuite blog, and I've picked a few of my favourites.
Picjumbo It's a nice resource but only has a small number of images (600 or so), but they are of excellent quality.
Unsplash is excellent - my search for 'books' returned a lot of results (277) and they were really interesting pictures.
Stockvault was another good site, and again I got a fair number of results for books, but the images were rather more boring to be honest.
Negative space gave me a few book images, but I hadn't seen them anywhere else, so was happy with that.
Kaboompics has a great name and some superb photographs.
Stocksnap has some really excellent images, and my books search gave me 145 really good quality and interesting images.
Splitshire had no books pictures :( but it did have some really fantastic images, so it's worth a look.
Life of pix has some really good very high quality and good resolution images. Only a couple of books pictures, but certainly worth exploring the database.
Pexels is a multisearch engine with access to almost 4,000 images with more added every week.
Jay Mantri is a photographer who has made many of his images freely available for people to do anything they want with them. No search option that I could see, but excellent landscapes and abstracts.
ISORepublic has free stock photos for creatives. Lots of excellent images.
Free Refe is a good collection of high quality images.