Nicholas Basbanes' first "book about books", A Gentle Madness, has been on my "to read" list for ages and I've finally tackled it (finishing the last third or so sitting on the roof of my apartment enjoying a wonderfully cool August afternoon). With a journalist's knack for a good story and a bibliophile's recognition of and respect for good books, Basbanes has created a massive yet fascinating compilation of book collectors through the centuries, accurately describing the important role private collectors have played in the transmission and preservation of cultural treasures.
From Samuel Pepys to Thomas Phillipps to John Larroquette and uber-thief Stephen Blumberg (and far beyond), Basbanes captures the essence of book collecting, whether done legitimately through the auction house (ah, to have been a fly on the wall at some of the great sales he documents!) or dealers ... or illegitimately through deceit and thievery (see Blumberg).
This is a must-read for any book-lover who has not yet had the pleasure - and Basbanes' excellent twenty-page bibliography is an excellent starting point for the "books on books" shelf you've been meaning to put together.