The need for good quality free to use images appears to be only matched by the number of different databases that keep springing up to provide them - usually as the loss leader into their commercial offerings. However, if you find the image that you want, and it's free, then all to the good. Here's a bunch of the latest:
Dreamstime. This has a small number of free images, and more can be purchased for about a pound each. Out of the 23,000,000 images available in total I found 16 tagged as 'books' and 12 as 'library' but while they were clear images and neat, they weren't particularly exciting. They do have a lot more which are 'royalty free' but that's not quite the same as 'free' since you still have to pay a fee to download them. You also have to register before you can download the free images as well.
Free Digital Photos. Generally the smaller version of an image is available for free, but larger versions require payment. However, there were almost 2,500 images for 'book' and 181 for 'library'. Large resolution images can cost between £5 and £15. I liked their selections - quite interesting and very different from the boring and predictable images you might imagine.
Free Images. Nice and straightforward to search, and 'book' gave me 1,000 images (photographs and clip art) which were free to use, with attribution back to the owner. 'Library' gave me 424 images, and these were also interesting and amusing, though a lot of the ones that I looked at seemed as though they would be more at home tagged as book, but there you go.
Free Range stock photos gave me 148 and 30 results, but I didn't find them very impressive, and a lot of the library ones appeared to be of catalogue card draws. The images also didn't look very sharp either, so I wasn't impressed.
New Old stock was great fun. It's a collection of vintage photographs 'from the vaults' with no known ownership and in public domain. Disappointingly there was only 1 book image and 0 libraries pictures, but its still worth a look. The search option is very easy to miss - it's in the top left hand corner.
Public Domain Pictures was a site that I liked. It described itself as "a repository for free public domain images. Download high quality HD photos or upload your own. Make money from your hobby, gain popularity and improve your photographic and graphic skills." I couldn't see how many images there were for my searches, but I liked the ones that I saw, and while they're free to use, you are encouraged to buy the photograph a 'cup of coffee' via their payment system.
RGB Stock has a very generous set of licence terms which allows you to do most things other than make money directly out of the images. I got 934 and 117 images for my two searches, and was pleased with the images that were returned - varied and of very good quality. You do need to register for free to download images though.
These are a few of the resources that I particularly liked from a larger collection of 53 resources in a blog post over on buffer, and if you found yourself with an appetite to look at more resources, head over to their post 53+ Free Image Sources.