Depending on the quality of a picture, the age of the specimen, environmental conditions, etc., a picture can be deceiving when trying to match with specimen in hand. The Peterson Guide also uses some different names such as Leucocoprinus for the more familiar Lepiota. This may be a bit confusing, but leads to a better understanding of naming schemes. This is a good guide if you like to flip through pages and are a visual person. It has no keys, but good descriptions.
Many mushrooms look similar when comparing transient characters such as color, size, etc.
Without a keyed approach, it's difficult to emphasize important characters such as spore color, gill attachment, etc. Some of the pictures also contain specimens which are beyond prime condition, and not very representative of the species. I guess that makes two problems. Still a good book. This opinion arises from the confusion and frustration of a beginner trying to make sense of a seemingly endless list of similar looking mushrooms.