Resources for 2nd Bureau stamps and postal history

Beecher, H.W. & Wawrukiewicz, A.S. (1999). U.S. Domestic Postal Rates, 1872-1999, Portland, Oregon: CAMA Publishing Company.
Wawrukiewicz, A.S. & Beecher, H.W. (1996). U.S. International Postal Rates, 1872-1996, Portland, Oregon: CAMA Publishing Company.

Brody, Roger S., The Gold Medal Collection, Shreves Philatelic Galleries, Inc., New York, New York, Sale 54, 2002. — A fantastic exhibit of 2nd Bureau essays, stamps, and postal history. The defining 2nd Bureau exhibit.
Cheshire, The Cheshire Collection of United States 1902-08 Issues, Robert A. Siegel, New York, New York, Sale 873, 2004. — A splendid collection that contained some of the missing items from the Brody exhibit.
Drucker Family Collection, United States Post Offices in China and Japan (part 2), Robert A. Siegel, New York, New York, Sale 855, 2003. — Amazing collection with some extraordinary 2nd Bureau covers.
Fisher, George Y., United States Postal Agency in Shanghai, China 1867 to 1922, Richard C. Frajola, Ranchos de Taos, New Mexico, 1998. — Excellent collection of U.S./Shanghai postal history.

APS — The American Philatelic Society
USSS — The United States Stamp Society (formerly the Bureau Issues Association) — If you enjoy US stamp and postal history this is a great organization for you!
Phil Bansner — Great site for postal history and literature.
James E. Lee — Extensive philatelic literature for sale.
Stamp Auction Central — Extensive stamp auction listing site.
eBay — Huge Internet auction site
The Swedish Tiger — Trying to identify a U.S. stamp, this site is an excellent resource.
Stamp Smarter — Lots of specialized help on collections (U.S. plating, registration labels, cancellations, perfins, tagging, etc.)

Fifteen cents of postage paid for three times the UPU rate (1.5 ounces) to Switzerland.