Each of my car BOBs has the Collins Gem version of the SAS Survival Guide by Wiseman, along with a magnifier. Each also has a set of knot cards and a set of shelter-building reference cards.

The Hesperian books (publishers of Where There Is No Doctor) are excellent for a survival library. So is Desk Ref. Less of a reference work, I'm partial to Lundin's 98.6 Degrees... for a primer on survival.