In reply to Doug:
I'll second that recommendation. Terrific book, easy to read and written by people who have been to many places hillwalkers and climbers frequent.
Depending on which area of the country you're most active in, you could see if there's a book about it in the Collins New Naturalist series. Though perhaps a little harder going if you haven't any background, they are often good. Dated but not so's they're worthless, far from it, and can be bought cheaply second hand. One word of caution; I really didn't get on with the revised version about the Lake District, the original is far better.
For geology, many people think Granite and Grit is a good book for the hillgoer. I didn't, for what I think are good reasons, but am happy to recommend it on the basis that others like it.
More if I think of other things.
T.
Post edited at 20:12