If you're looking for a smaller book, then I can recommend this one highly:
http://www.amazon.co.uk/Field-Guide-Mushrooms-Britain-Europe/dp/1845374746/ref=pd_sim_b_img_8
I picked it up a wee while ago while flicking through pocket guides in a local bookshop - it has several things going for it.
Firstly, it has really good photos and illustrations - I find a lot of books are far too vague with their pictures, or have drawings that show the 'ideal' specimen, not the one you actually find on your travels.
Secondly, it has a main picture and 3-4 smaller pics of the important details, with numbers in the text that match the pictures. So the smaller pics might have a close-up of the stem, the gills, the environment, or the fungus in a younger or older form.
Thirdly, its cheap
Of course it has all the usual things like edibility markers, seasonal availability etc.
Of course, being a pocket guide it doesn't have everything, and if you're looking for a book to identify everything you find, you might want something a bit more detailed. However, if you want a book that will help you distinguish between similar fungi, or help you learn the characteristics of the more common edible, inedible and poisonous fungi, then this is the book for you.