Hi Jack and welcome to BCUK!
Most of the guys here use one of the Collins Gem field guides, or "Food for free" (Richard Mabey). Though these don't cover all the uses of a plant (mostly aimed at edible, and sometimes medicinal properties). Thus not what wood you could use best for a ember fire, or what bark to use to make cordage, etc.
Also have a look at this thread:
http://www.bushcraftuk.com/community/showthread.php?t=8843
The internet also is a big resource, drawback would be the fact you can't take it with you in the field
This thread has some interesting links to websites:
http://www.bushcraftuk.com/community/showthread.php?t=5711
If you're interested in a field guide that covers all aspects and usefull parts of plants, I suggest you read my post I made here previous:
http://www.bushcraftuk.com/community/showthread.php?t=11823
The idea is simple: go for a good ID book (probably one you've already found) and add the other info (about usefull parts, edibility, toxicity, etc) yourself, either by writing in the Flora
(blasphemy
) or use Post-its (though ... add the page number and name of the plant on it ...
), or use a seperate note book
.