In my experience there'll never be just one! (I have something in the region of 60 & counting). On the other hand, something you can slip into a pocket while you're out and about is useful. Collins Gem editions are quite good for portability & quick reference - there is a 'Food For Free' version and individual titles for fungi, trees, flowers etc.
What you'll find is that there are books that are good for id that don't tell you much about edibility, and books that cover edibles that aren't really designed to help you with id (they usually assume if you've got as far as eating something you should know for sure what you're looking at). Because of this I have yet to find a really definitive book and usually resort to cross referencing between the good id guides & the actual uses/recipe type works to suit my personal needs & interests.
I'm sure most folks will point you at Food For Free as a good starter but it's not the only book on the topic & I think it's likely that you'll quickly outstrip what it can teach you. Neil Fletcher's Easy Guide is quite good and possibly Wild Food For Free by Jonathan Hilton. I suggest starting out by trying your library to see what they have that you can look at before you commit to buying anything and ask if they can order in titles for you. Or have a quick flick through the appropriate section at Waterstones or Smith's. Don't forget to have a good trawl around charity shops/bootfairs/second hand booksales too, you never know what you might come across. You can check out Amazon for a rough idea of current titles and their availability & pricing as there has been a rash of new stuff coming out in the last year or so.
Books I've personally found useful and consider worth a look would include anything by Roger Phillips, Claire Loewenfeld , Pamela Michael, Hugh Fearnly Whittingstall & the River Cottage guides & any of the Reader's Digest plant guides but that's just my opinion.
A good book to start with
