h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A really good place to find first editions is Oxfam. Most shops have a volunteer who is traied in such matters and as a result a lot of the the "old but not valuable" books are, erm, "recycled", cutting down the chance of buying a lemon, as it were.

There are quite a few specialist Oxfam bookshops around, but pretty much all shops will sell some pretty interesting books.

I am biased towards Oxfam, but I would assume that most of the charity type shops around will have a similar setup. Try it - you might land a bit of a bargain!