This place is "Book Porn" in the strongest sense of the word.
I don't mean in some twisted, dirty "Adults Only" sort of way. But if you are a fan of the printed word, this place is worth the visit.
It's history, pure and simple. Books date centuries back in this place, and are on display. The service has always been very nice, and they have catalogues that you can take home and browse through the selections offered.
Bear in mind, that this is a collector's place. You don't plan to buy these books for cheap (think thousands upon thousands for some of these first editions), and these aren't something one would place on a standard bookshelf and go. I wouldn't even begin to speculate the care needed to display these books, much less a letter from Abraham Lincoln (which they have).
Still, it's an amazing place to browse through and see what was. And if you love books, Bauman Rare Books is utterly fascinating.