A Book is a collection of document files. It's a useful method of organizing multiple separate files into a longer publication. A Library is a collection of asset objects you can place into other files. It's commonly used to hold assets that are used over and over to speed up your work.
Does 'asset objects' mean something like the graphics and texts? or what?
Can be just about anything. A text frame, a photo, a group of objects, an entire page layout...
I seldom use libraries, but I have one annual directory project where I have a library that contains all the section headers in frames, along with some fillers, so I can just drag them from the library as I go along so I don't need to create them fresh each year. Because the listings vary and heading positions are dynamic I can't just set up master pages to handle it.