Now that I have a camera and carry it all the time, I also take many pics of plants, but, as Wayland said, I feel I really want to know there and then - and for excellent reasons:
When I get home and look at the photos with my books, in the key you are asked whether the leaves are opposite or alternate (for example) - check the photos, and of course I can't see because the photo is only of the flower and the tops of the leaves. The keys require a far more detailed examination of the plant than one can make with a camera.
With the photos you can resort to simply leafing through all the photos in the books to see if you can get a match, but that seems to be worst approach since one is not learning about the botany of the plant - the family, its features etc.
I don't have the Collins Gems nor their pocket books, etc. so I will have a look at them since they seem to be used by so many of you
The Roger Phillips books I'll look at extremely critically. They are photographer's books and I have just burnt my fingers with the books of Bob Gibbons, who is also a photographer, because I discovered that many of the photos which look so nice are pretty useless for identification - and some of them are just plain bad, such as "over there" shots, where the plant in question is "over there" and one can barely make out more than blotches of flowers. The publishers/printers may to be blame, of course, but I am now very much in favour of colour drawings based on photographs.
Of the books that I have, only two were mentioned by any of you though some of my books are French, Dutch etc.
Of the books that I have, the one which I think would be best outside, is Rose's "The Wild Flower Key" which is at the top of Scrubcutter's list and the Hamlyn "Edible and Medicinal Plants" by Launert which Xylaria mentioned - but neither of these books is used by anyone in the field!
When it comes to
which book people really do take out with them, it seems that the Collins publications, in particular the Gem series, are the favourites.
Oh well... back to the bookshop "Excuse me do you have the Collins Gem series..."