I try to keep my reference books to a bare minimum these days. I find anatomy books useful and I have a few, but generally I prefer to take reference photos, usually of myself or some other unwilling subject I've forced into an awkward pose (10 minutes before they're about to leave for work). If i need something specific, I used to use google images, but these days I usually head to Flickr. I also shy away from "art" books now too, I have a few prize ones that would be impossible to replace, but at some point I realised that 90% of them were gathering dust, and most of them were purchased just so i had something to buy....consumerism, tsk, so i sold them.
So on that note, I picked up DK's Weapon at the weekend. It's strange, but it's inevitably the books you hum and ha over in shop that become indispensable when you get them home. This is a great visual encyclopedia of weapons, perfect for the budding fantasy artiste like myself. I'll be picking up Soldier at some point too (or Warrior as it's known here) for the clothing and armour, probably when it comes out in the compact edition (because like everything I want to buy in Australia, it's about double the price I would pay in the UK).