What the plan is to have all of the books worth carrying with me, and to also start making my own refrence. Any one who Bird watches will tell you it is a nightmare to find a decent single refrence.
It would be easier to do in a database or other form, but in html it basically runs on anything.
The best herb book is a version of Culpepper's complete herbal. You can get the original from works and similar. The one people recommend is the colour one I think, which is availible on Amazon. I am sure someone will put up the full title.
There are a couple of tree books out of Kew, that are surposed to be good reading, and contain lots of info.
Have a look for your local Botanical gardens, they tend to have a lot of experts who do walks and things. Plus they will have native species labeled up for you to look at. There is a garden owned by them, called the scienific garden I think somewere london way, that has every single herb, and medical plant labeled. Worth checking around, Arwick castle has just opened their new gardens, and I am sure they have something similar.
Books are good, but I still spot trees the way my scout leader taught me. If you can find someone who can teach you the little tricks of the trade it is so much easier.