Best Field Guide for Positive I.D.???

[Don Wheeler]

I am relatively new to mushroom identification as a hobby.  I've been 
using the Audubon Field Guide to North American Mushrooms for a couple of 
months and have few complaints--however, there have been a couple of 
incidents in which I absolutely could not make a positive ID. Are there 
any other field guides or alternate resources that I need to add to my 
collection--keeping in mind that I'm primarily interested in North 
American species.

				Any info is greatly appreciated.
				Thanks.