Typically bills go through various stages on their way to becoming law. As Congress goes through the legislative process, various documents are created that can serve as useful sources of information.
Bills - might be different versions
Committee Hearings - includes prepared remarks that are included in the record and testimony
Committee Prints - these are usually research reports on the topic. Not all bills have an accompanying committee print.
Committee Reports - When a committee decides to recommend a bill to the whole House or Senate they will produce a report explaining why the committee members believe this bill should become law. Federal courts often use these reports as a way to figure out what Congress intended when they passed a law.
Congressional Record - reports what is said on the floor of the House and Senate during debate.
Statutes at Large - a compilation of the laws passed in a particular congressional session.
U.S. Code - laws are broken up by topic and inserted in the relevant sections of the Code. Many laws deal with multiple topics.
Legislative Histories - tracks the various stages and documents that are related to a particular law. These are great sources for finding the relevant documents. Often these legislative histories cover multiple congresses.
Congressional Research Service (CRS) Reports - the CRS conducts research on a wide range of topics at the request of members of Congress. The reports can be quite valuable.