When you have a book file and at least some of its chapter files (plus possibly some unrelated files) open, you have the choice between saving (or closing) all files in the book or all open files (including the book file and other files that may be open). Both use the same Shift key, which changes some entries in the File menu.
If you have the bookfile selected, Shift + File menu brings up the extra commands "Open All Files in Book", "Close All Files in Book" and "Save All Files in Book". If you have any other file selected the same Shift + File menu brings up the command "Close All Files" and "Save All Files".
Oh, I should add: if all open document windows are minimized, the commands are disabled. You need to have at least one window in non-minimized state. The commands do work on all files, including those with minimized document windows, though. This is a little bit awkward - probably nobody ever tried this at Adobe.
Ciao from Amsterdam
Jangli