I usualy have a Colins Gem of Trees, although at this time of year it doesnt do me much good. I also have the Colins Gem of wild flowers, and the Small SAS Survival Guide.
I usualy also take the Ciserone "wildeness medicine" book. This one isnt full of bushcrafty type potions but is a standard medical aid for when you are away from civilisation.
Hmmm..I suppose that is quite alot of books isnt it...no wonder my sandwiches always feel heavy.