In reply to Cobra_Head:
> They're a bit big for you pocket but I love the Collins Guide to xxx series, the Insects book is my favourite book of all time.
Yes, I think the Collins Field Guides are pretty good for the interested non-specialist. Detailed enough to identify most things you're likely to see (or at least something obviously closely related) I have half a shelf of them covering invertebrates, pond life, snails, spiders. birds, reptiles, trees, ferns, fungi, wild flowers etc, UK and Europe.
If you really want a thing of beauty and authority on fungi, then it has to be something by Roger Phillips.