136 Points of Reference is highly recommended and also happens to be very much in the spirit of this blog. The editors of Folk Archive confess that "for everything we have included there are a hundred objects or events that we have left out through ignorance or our own preferences". I feel the same way about Reference Library. The Herbert Collection: Works and Documents is another one; I've only flipped through it once, but I would like to have another look. The Spirited Art of Sister Corita never fails to inspire. The Mennonites is nice, but not great. Jessica Stockholder has a good selection of her installations and sculptures, which I like very much. Yayoi Kusama is awesome. The Complete Rhyming Dictionary "comes in handy," says Andy.
And that Letterpress book is also very nice.