Most researchers looking to understand older government contracts are looking for values and products/services rendered. However, there are times when understanding the process and general contracting environment and system can helpful in understanding how data is reported and where to find it.
The changes in government contracting have mostly changed by a series of laws as well as the implementation done via the Federal Acquisition Regulation (Title 48 of the CFR). There are a number of laws that have influenced federal contracting but much of what happens today is a result of the Competition in Contracting Act of 1984. CCIA was a significant overhaul of federal acquisition law largely intended to increase competition from contractors to improve federal procurement outcomes. Below is a list of just a few of them.
As for finding individual contracts, academic and trade literature can publish articles that are general about contracting with the government but they can also do articles on very specific issues like defense contracting or looking at a particular time period or product. For example, the Economic Priorities Report published "The Business of War: 523 Corporate Contractors for the War in Indochina" in their February/March 1971 issue. One other source to mention is Dun's Review. This general interest business publication includes articles on a variety of subjects, but it also covered government contracting when necessary. During World War II, for example:
Historically speaking, spending by the War Department / Department of Defense has accounted for the bulk of government contracts which is why so many of the sources like the Quartermaster manuals included below, focus on that area. We have included more general titles but there are publications that address more specific situations like Construction in the United States (1972) or Buying Aircraft: Materiel Procurement for the Army Air Forces (1964) or articles like "Business of War: 523 Corporate Contractors for the War in Indochina" (Economic Priorities Report Vol. 1, No. 7 - Feb/March 1971). One area also not included but which can be helpful, are congressional documents like the notable hearings of Admiral Rickover on Defense Contracting (Joint Economic Committee Jan. 28, 1982).
When it comes to understanding military procurement, how the department has been organized plays an important role in how contracts have been issued, recorded, and stored so below are four of the most major pieces of legislation that changed defense organization.
The following titles link to fuller bibliographic information in the Library of Congress Online Catalog. Links to additional online content are included when available.
Arming the Eagle by 
Army and Economic Mobilization by 
The Business of Civil War by Defense Acquisition Reform 1960-2009 : An Elusive Goal by Doing Business with the Federal Government by Final report[s of study groups, etc.] for the Commission on Government Procurement by 
Freedom's Forge by Government contract problems, prepared by the War Economics Division of the Research Institute of America; Leo M. Cherne, editor-in-chief, William J. Casey, chairman by Handbook of War Production by A History of Government Contracting by History of Military Mobilization in the United States Army, 1775-1945 by Instructions Governing the Procurement of Supplies and Engagement of Services, Quartermaster's Department, U. S. Army
The Military and the Market by Mobilization : the U.S. Army in World War II--the 50th Anniversary by 
The Neck of the Bottle by Prentice-Hall National Defense ServiceProcurement Handbook by Quartermaster and ordnance supply; a guide to the principles of the supply service of the United States army, by instructors of the Army supply service course, the University of Chicago by The Quartermaster Corps, Organization, Supply, and Services by Recommendations of the Commission on Government Procurement : a final assessment : report to the Congress by United States Army Logistics, 1775-1992 : an Anthology by The War Production Program. Selected Documentation on the Economics of War by War Production in 1942 by Wartime Purchasing Procedures, prepared by the War Economics Division of the Research Institute of America. Leo M. Cherne, editor-in-chief, William J. Casey by 
Defense Indicators by Historical Reports on Defense Production by Statistics of War ProductionSummary of War Supply and Facility Contracts by State, Industrial Area and County. by 
