JSPs and servlets have become rather popular in the last year or two and are discussed in a large number of books and magazines. O'Reilly and Associates has published two books on the subject: Java Servlets and Writing JSP Pages. The former is an excellent guide to servlets, while the latter came out just before I wrote this column, so I have not yet obtained a copy.
There is plenty of on-line documentation for Java servlets. While it was difficult to find good, well-written documentation for Tomcat just 12 months ago, there currently seems to be an abundance. The Jakarta web site at http://jakarta.apache.org/ is a good starting point; with the Tomcat minimalist user's guide (http://jakarta.apache.org/tomcat/jakarta-tomcat/src/doc/uguide/tomcat_ug.html)
and mod_jk HOWTO
(http://jakarta.apache.org/tomcat/jakarta-tomcat/src/doc/mod_jk-howto.html) are probably the best examples.