The gold standard in class-action scholarship for practitioners is Newberg on Class Actions, the 4th edition of which also includes Survey of State Class Action Law by the ABA's Section of Litigation.
Newberg is a very comprehensive treatise and should always be consulted on difficult issues. Unfortunately, the book's comprehensiveness sometimes makes it hard to find specific information. I have also seen instances in which past editions contained text relying on cases that had been overruled long before. This problem has probably been corrected with the latest edition, but be sure to shepardize.
The publications by the National Consumer Law Center are also very well done, although their scope is more limited. Class actions practitioners, whether for the plaintiff or defense, might find value in Consumer Class Actions and Unfair and Deceptive Acts and Practices.
All of these books have materials of interest specifically to Illinois lawyers. In addition, the Illinois Institute for Continuing Legal Education (IICLE) publishes Class Actions, which was updated in 2004.