I know of four main sources for IPO information: SEC filings; Thompson Financial Securities Data; news; and analyst reports.
SEC Filings: Companies about to do an IPO must file an S-1 registration and a Prospectus with the SEC.
You can pull S-1 filings for free with the SEC's EDGAR Company Search, and you can search for S-1s from the past four years for free with the Advanced Full-Text Search. For better searching, use one of the commercial SEC research systems discussed in the Filings section of the "Securities and Exchange Commission" entry.
Note: There is lots of information in an S-1, but companies generally try to be as vague as they think the SEC will let them get away with.
IPO Research Services: You can do basic IPO research for free using EDGARonline's Initial Public Offering (IPO) Search, but make sure the years you need are covered. Subscription services include Lexis Securities Mosaic, Capital IQ and Thomson ONE (formerly SDC Platinum and the and Global New Issues database). You can also place an order for IPO research request by calling ThomsonReuters's Customer Support Line (888-989-8373).
News & Related Information: Hoover's IPO Central provides lots of information on IPOs, including data on recent IPOs and filings (with a 24-hour delay, unless you are a paying customer). Also check out Yahoo!'s Recent IPO News.
Analyst Reports: Brokerage houses issue reports on IPO companies (to get these reports, see "Analyst Reports"). However, you will not find reports issued while the IPO underway because the SEC prohibits brokers from commenting on IPOs from the time of filing until a month after the IPO is completed.
Treatises: Treatises discussing the practical and legal aspect of IPOs include
Initial Public Offerings: A Practical Guide to Going Public (PLI; more info here) and the
How to Prepare an Initial Public Offering (PLI), from the Corporate and Securities Course Handbook series. Venture Capital & Public Offering Negotiation (Aspen) covers how to take a Portfolio Company (i.e., a company receiving venture capital funding) public.