The official source for the laws and resolutions passed by Congress. Each year, every law passed by Congress is published in an annual volume.
To find laws about apportionment, go to the index of a year (or the cumulative index for the first 8 years) and look up Apportionment. The number there refers to the page number - note it. Then go to the "Title Page" for the volume and type in the page number you want to visit.
Or, if you have a citation, for example, just go to volume nine, click on "Title Page" and type in 432 as the page number.
The Congressional Record (and its three predecessors listed below) is the official record of the deliberations behind lawmaking, from speeches given on the Senate and House floors to "extensions of remarks," additional documentation that lawmakers formally add to the record even if not spoken out loud.
The Journals of the House and Senate are considered the minutes of floor action. Unlike the Congressional Record with its full debates, the Journals just summarize the action in each chamber. This online version links from the Journal entries to related full text transcriptions in the Register of Debates, the Congressional Globe and the Congressional Record.
The U.S. Serial Set (and its predecessor the American State Papers) is a collection of House and Senate Reports and Documents including congressional committee reports on pending legislation, noncongressional materials ordered printed by Congress such as special studies and background information, annual or special messages to Congress, and presidential messages to Congress.