I have near a full set of the Double Gun Journal, so volumes I & II of the index and reader are nice reference books. Lots of good stuff that goes beyond the guns and visit hunts, craftsmen, fields, dogs, etc.
I've long since given up on any of the pricing guides. It seems I'll flip through them a bit when they're new, then never even glance at them again, but I seem to regularly go to my set of Brownells Gunsmith Kinks.
I picked up a copy of Venters Gun Craft, basically to ride along with another order for favorable shipping rates. Not a big name classic, but for a newer book, I'll admit that it gets my attention with a bunch of interesting tidbits strung throughout it.
So much good stuff. I need to get off my tail and set my little library nook that's been on the to do list for a bit.