There a few decent books on the market at the moment, try looking in a big bookshop like Waterstones where you can browse properly and get a feel for the different types. Food for Free is the one that's most frequently mentioned but it isn't the only one (nor is it the best IMHO).
It all depends on how deeply you get (or want to get) into foraging. I have literally dozens of books on the subject and much of the footwork is actually research rather than out in the field (because it's always better to know what you're picking before you pick it!).
I'd recommend any of the Collins Gem field guides for portability and clarity, and once you've double checked your details you can always add colour coded annotations to give you an at a a glance guide for edibility/toxicity which is what I do.
Do loads of research and get into the habit of learning latin names too if you can, it's invaluable for checking identification especially where a name alters from region to region. Read everything you can lay your hands on, ask questions, use your eyes and learn to be observant
whereever you are. Take notes if you want and pictures too, there are loads of websites where you can post pictures and details and get opinions on identification and advice.
The main rule with foraging is, if in doubt,
don't. But once you've done your homework there should be less doubt and more do.
Good luck!
