What is the difference between 'Document', 'Book' and 'Library' in InDesign?

[domadotomato]

I often use 'Document' in making files such as brochures and also an ebook. But I never used book nor library when I make a new file. When and where (what type of file) should I use document, book and library? What is there significant difference?

[Peter Spier · Accepted solution]

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.

[domadotomato]

Does 'asset objects' mean something like the graphics and texts? or what?

[Peter Spier]

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.