There are not a tremendous amount of sources on the history of Pennsylvania Avenue. Certainly the best place to start would be the National Park Service website for the Pennsylvania Avenue National Historic Site. The website is a bit dated, but provides a good overall history of the Avenue.
Newspaper articles also provide a great deal of information about the Pennsylvania Avenue Development Corporation (PADC) and its predecessors. Historic Washington Post articles can be viewed with a subscription through the Proquest Research Library. Articles would also be in the defunct Washington Star, but the Star is not available for searching on-line.
Certainly, the special collections at George Washington University's Gelman Library are the best concentrated source of information on the redevelopment of Pennsylvania Avenue, specifically the collections from the Committee of 100 on the Federal City, and the Metropolitan Washington Region Urban History, papers.
There are very few books specifically about Pennsylvania Avenue, but numerous books and on-line resources about sites along the Avenue are accessible. Below are just a few suggestions to get you started.
Evelyn, Douglas E., and Paul Dickson. On This Spot: Pinpointing the Past in Washington, D.C.: Farragut Publishing Company, 1992.
This book is a historic site guide book on the District of Columbia. Chapter 3 is titled "Old Pennsylvania Avenue - America's Main Street," and describes some of the more common as well as lesser known historic sites and events.
National Park Service. 1997. Pennsylvania National Historic Site. (accessed 4/29/2006).
This National Park Service website provides a good basic overview of the Pennsylvania Avenue National Historic Site. The website is almost ten years old, so it is not terribly high quality, but it is still a good starting point.
Highsmith, Carol M., and Ted Landphair. Pennsylvania Avenue: America's Main Street. Foreward by Daniel Patrick Moynihan. Washington, D.C.: The American Institute of Architects Press, 1988.
Although slightly dated, this is probably the most recent, and one of the few works, specifically addressing Pennsylvania Avenue. Although it is not tremendously detailed, it is an excellent overview of the Avenue's life and history. Probably the best feature of this book is its vast assortment of images used.
Gurney, George. Sculpture and the Federal Triangle. Washington, D.C.: Smithsonian Institution Press, 1985.
This is an excellent source, not just on the decoration of Federal Triangle, but on the architecture itself. It begans in the planning stages, and takes the reader all the way through the construction. The book concludes with the sculpture and artwork.
Allen, William C. History of the United States Capitol: A Chronical of Design, Construction, and Politics. Prepared under the direction of the Architect of the Capitol. Washington, D.C.: U.S. Government Printing Office, 2001.
This book is the definitive work on the history of the Capitol building. It also addresses the issue of designing Washington, D.C., and, to a small degree, Pierre L'Enfant's intent for Pennsylvania Avenue. This book is available, in its entirety, online at History of the United States Capitol
D.C. Preservation League (2001-2005)
The D.C. Preservation League orginated in 1971 as "Don't Tear It Down," in an effort to preserve the Old Post Office building. Since then, they have struggled to save a significant number of historic sites throughout the District of Columbia.
Old Post Office Pavilion (2006)
This is a website for the current incarnation of the Old Post Office building. It includes information on usage of the building, a brief historical essay, and information on touring the tower.
Most of the documents in this module were drawn from the records of the Committee of 100. This link is the Committee's official website. Although it provides a fair amount of information, it does seem to be a bit behind in updates.
Committee of 100 on the Federal City, records (Part I), Special Collections, Gelman Library, George Washington University, Washington, D.C.
These are the compiled records of the Committee of 100 on the Federal City. Most of the documents in this module were drawn from this collection. The collection is an invaluable resource on the activities of both the Committee and the Pennsylvania Avenue Development Corporation.
The Metropolitan Washington Region Urban History, papers, Special Collections, Gelman Library, George Washington University, Washington, D.C.
These papers focus on the urban history of the District of Columbia, and do have some limited information on the redevelopment of Pennsylvania Avenue.