As they say, you never have too many mushroom books.  I started with one mushroom field guide,
Field Guide to North American Mushrooms  by Gary H. Lincoff (1981).  In a little over 5 years I have managed to gather the small library listed below.  They are in the approximate order of use.

Field Guides
Field Guide to North American Mushrooms  
by Gary H. Lincoff (1981)
Mushrooms of Northeastern North America by Alan E. Bessette and others(1997)
North American Boletes by Alan E. Bessette and others (2000)
Mushrooms and Other Fungi of North America by Roger Phillips (2005)
Mushrooms of Northeastern North America by George Barron (1999)
Mushrooms of Cape Cod and the National Seashore by Arleen R. Bessette and others (2001)
Mushrooms Demystified by David Arora (1986)
The Mushroom Hunterís Field Guide by Nancy Smith Web and Alexander H. Smith (1996)
Mushrooms of North America by O.K. Miller Jr. (1977)
Simon & Schusterís Guide to Mushrooms by Giovanni Pacioni and Gary Lincoff (1981)
The Encyclopedia of Mushrooms by Colin Dickenson and John Lucas (1979)
Mushrooms of the World by Giuseppe Pace (1998)
Eyewitness Handbooks Ė Mushrooms by Thomas Laessoe and Gary Lincoff (1998)
A Field Guide to Southern Mushrooms by Nancy Smith Weber and Alexander H. Smith (1988)
The Mushroom Handbook by Louis C.C. Krieger (1935)
Field Guide to Wild Mushrooms of Pennsylvania and the Mid-Atlantic by Bill Russell (2006)
North American Mushrooms by Dr, Orson K. Miller Jr. and Hope H. Muller (2006}

General Reading
Celebrating the Wild Mushroom
by Sara Ann Friedman (1986)
Mycelium Running: How Mushrooms Can Help Save the World
by Paul Stamets (October 15, 2005)
Morels  by Michael Kuo (2005)
One Thousand American Fungi: Toadstools, Mushrooms, Fungi : How to Select and Cook the Edible : How to Distinguish and Avoid the Poisonous
by Charles McIlvaine (1973)
All That the Rain Promises, and More... by David Arora (1991)
Medicinal Mushrooms by Christopher Hobbs (1986)
Mushrooms in their Natural Habitats by Alexander H. Smith, Ph.D. (1949)

Microscope Books
How to Identify Mushrooms to Genus III: Microscopic Features
by David Largent and others