The Americans with Disabilities Act is not the least complicated law on the books. Employers have a lot of questions, and they need answers to stay within the letter of the law, and avoid the courts.
Here are a number of excellent sites online to begin your research on the ADA.
-- The government has its own site for employers dedicated to the ADA, called the ADA Home Page of all things.
-- This site contains a full copy of the ADA, annotated with changes made to the law since 1990.
-- And from the Office of Disability Employment Policy with the Department of Labor comes the Job Accommodation Network, a free consulting service meant to increase the employability of disabled folks.