A mook is a printed publication that is considered to be half magazine / half book. It's usually less 'book-like' than a book in that it resembles a magazine, but is a higher quality magazine, i.e. doesn't feature (much) advertising, has nicer paper stock and a thicker, glossier cover.
A Mook is not as disposable as a magazine, but not as large as a book.
You treasure a mook - you dispose of a magazine.
You collect mooks - you throw magazines away.
You poor over the content in a mook - you scan through magazines.