The Government Accountability Office (GAO) is an independent, nonpartisan agency that works for Congress. Often called the "congressional watchdog," GAO investigates how the federal government spends taxpayer dollars. While the focus of the reports are on fiscal analysis, they do provide excellent background information on policies that can be helpful to your overall policy analysis. Background information includes a review of literature, related laws, regulations, etc.
The Congressional Research Service (CRS) is a non-partisan public policy research arm of the United States Congress. CRS provides reports to members of Congress on a variety of topics relevant to current political events.
Congressional Committee Reports are part of a set of documents called the US Serial Set. House and Senate Committee Reports contain the findings of committee hearings or the outcome of committee deliberations. They can contain discussions of legislative intent, a short history of a bill, and comparisons of current and proposed law text.
For more information about Congressional Reports and Documents, see this guide from LexisNexis. Congressional.