It really depends on what area of j2ee  you are interested in, j2ee is
pretty large.
Are you sure you don't just mean books on java  in general...... Do you
know the difference

anyway some books that come to mind on j2ee:

Core Java Servlets now in second edition
and it's followup More Java Servlets both from Sun press
(first edition and bits of second edition are available free
http://www.coreservlets.com/#Books)

Java Enterprise in a NutShell from O'Reilly -
http://www.oreilly.com/catalog/jentnut/

A few chapters of Thinking in Enterprise Java are available (it's not
all written yet) at http://64.78.49.204/