Library of Congress Research Guide: Doing Company Research: A Resource Guide
There are also reports, available for a fee, which are prepared by investment houses like Merrill Lynch, Credit Suisse First Boston, Morgan Stanley Smith Barney, Deutsche Bank, etc. These reports are on public companies only, and can be quite expensive from aggregators like "Investext" for those that subscribe. Some reports may be available from "Business & Company Resources (Gale publishing)" a product often found in local public libraries