The leading legal treatise on U.S. class actions is Newberg on Class Actions (Lawyers Cooperative Publishing). The Survey of State Class Action Law (ABA Section of Litigation) summarizes the laws of each state.
Information on recently filed and settled class action lawsuits is available free from Class Action World and Class Action Litigation.
Information on securities-related class action suits is available through the Securities Class Action Clearinghouse, sponsored by the Stanford University School of Law. The Clearinghouse posts complaints, opinions, orders, docket sheets, settlements, etc.
Periodicals reporting on key class actions include Stafford's Class Action Law Monitor, BNA's Class Action Litigation Report, Mealey's Litigation Report: Class Actions, and Thomson/West's Class Action Reports.
The Advisory Committee on the Federal Rules of Civil Procedure posts sample notices to send to potential class members.
The law firm of Milberg LLP, which handles many of the biggest class action suits, posts information about their cases.
You can find class action lawsuits involving a particular company in Federal court using Courtlink (search in Federal District Courts and select "Class Actions Only") or Westlaw (DOCK-DCT-ALL database; under "Case Type" select "Class Action)."
To learn about more class action materials, see Keeping Up with Class Actions: Reports, Legal Sites and Blogs of Note by Scott Russell.