I recently bought a Collins Nature guide on Mushrooms and so far am very impressed with it, small enough to carry about, has full colour photographs, a decent key and perhaps most importantly of all it gives the spore colours which is one place where the Collins Gem book is woefully inadequate.
One point that a fungus expert* I spoke to recently made was that when picking, its handy to unearth the bottom of the mushroom stalk with a culinary fork. That way you get to see properly of its got a volva which identifies it as an Amanita, a genus that contains many pretty nasty mushrooms.
Made a lot of sense to me and made a lot more sense than the practice of cutting the stalk.
*As he was running the mushroom identifying course I went on I'm calling him an expert.