The main U.S. law concerning disabilities is the "Americans With Disabilities Act." The Act is codified at 42 USC 12101 et seq. (see "United States Code").
The Department of Labor has an ADA Information line (1-800-514-0301). Also the Disabilities and Business Technical Assistance Center's library will provide information (800-949-4232).
The Americans with Disabilities Center posts the Act, plus related regulations and documents.
The multi-volume Individuals With Disabilities Education Law Report (LRP Publication) compiles federal statutes and regulations, IDEA policy rulings, Section 504 policy rulings, federal and state court decisions, and SEA appeals decisions.
The Department of Justice Web page posts ADA regulations and DOJ enforcement and settlements on its ADA Home Page.
The Social Security Administration posts information on Benefits For People With Disabilities.
You can link to ADA-related materials through the Federal Equal Employment Opportunity Laws, Cases, and Resources page in the LLSDC's Legislative Source Book.
The legislative history materials for the Individuals with Disabilities Education Act (Public Law 101-476, as Amended by Pubic Law 102-119) is available in the U.S. Federal Legislative History Library on HeinOnline.
Treatises: Good legal treatises discussing disabilities law include Employment Law Guide to The Americans with Disabilities Act, the multi-volume Disabilities Law Reporter and CCH's Accommodating Disabilities.