My favourite fungus book is Roger Phillips's
Mushrooms, but it's a bit of a lump to carry round, and I wouldn't want to get it wet. However, there's a good app which is derived from it, called
Rogers Mushrooms, which is available for iPhone and probably other flavours of phone. Its advantage for field use is that you probably have your phone to hand when in the wood, and it doesn't need a data connection. There's a free version, with a limited dataset, and a pro version which is a snip at £2.49.
One other option, which I've used a couple of times, but you need a strong data signal and a bigger screen than most phones have, is
MycoKey Online (
http://www.mycokey.com/newMycoKeySite/MycoKeyIdentQuick.html). It comes from a couple of Danish academics, it's free (in the version referenced), it's interactive, it's comprehensive, and it's simply superb for fungus identification.