what is it? how does it work?

It's a way for members to exchange several books at once (saving a lot on postage costs) without using credits.  It costs $8 a year.  If you go to the help help center and type in "box of books" in the browse help docs search window, there are some good help docs that go into details.

FYI: you can search other Boxers shelves before you join to see if people have books you would want to trade for or not.  And if you join don't feel guilty if you turn people down.  When I was a boxer I never did trades for books I didn't want just to be polite.  To me that defeats the whole purpose of doing box of books to save money on shipping. Just politly say no.