Hi SteveW,
As a practising herbalist, I'd say Julie Bruton Seal and Matthew Seal's book 'Hegderow Medicine' would be a great place to start. Redneck says its basic and I suppose as a book aimed at the general public, that is true, but I think it's better than that - not only does it give you the properties of common wild herbs, but it also tells you what you can use them for, how to use the with some excellent recipes. You can learn all the basics preparation techniques here - how to make teas, decoctions, tinctures, ointments, lotions etc. The herbs contained are all fairly safe to use and where there might be issues to be aware of, such as with Colts foot, it clearly tells you so.
i know Julie every well - she's a colleague of mine at the Association of Master Herbalists - she's an excellent herbalist and will have tried all the recipes out herself many, many times. No just copying information and recipes from other sources which you will find in some books. She's also a great photographer and most of the photos used are her own. These are a great starting point in helping you identify the plants in question but please be aware that this does not claim to be a plant ID book - you will need other guides to help you there. Note: I have no financial interest in recommending this book.
when it comes to using herbs there are some things to bear in mind:
1. Positive plant ID - if you're 99% sure, that's not good enough. Get to know the plants well before you start to use them.
A good starting point is to use herbs that are also foods - like Chickweed, Dandelion, Cleavers - these tend to be very safe but can still be very effective.
2. Know what you're treating - seems obvious but if you're not sure what the problem is, how can you hope to treat it. if you're not sure see your GP or a qualified herbalist.
3. Start simple - remedies for colds and flu, ointments for scrapes and bruises - don't try to tackle complex conditions without guidance. That's not to say don't use herbs for difficult conditions but do get some expert advice before you do.
4. Even the safest herbs can cause allergic reactions in some people. start by using just a little and see how you react to it.
5. Don't be put off if 1-4 seem a little intimidating. I started off by being fascinated by the healing properties of wild plants as a lad and started making teas and decoctions and eventually trained as a herbalist. As your experience grows so will your competency and confidence.
So so give it a go Steve, it's a fascinating subject and you never stop learning. It also makes every walk in the countryside even more interesting as you take more and more notice of what is growing around you and what you can use it for (just as foraging for food does). Very best wishes with this and if I can help in any way, let me know.