As servlets and JSPs become increasingly popular, there are more and more resources available for those who want to learn about them.
The site servlets.com, maintained by Jason Hunter (author of Java Servlet Programming, published by O'Reilly and Associates), contains quite a bit of information about servlets, much of which is useful to JSP authors. Even more interesting, however, are two articles describing “the problems with JSPs”. While I disagree with some of his points, there are definitely some good ideas and things to keep in mind, particularly if you are unsure of whether to use servlets or JSPs.
An excellent printed resource on JSPs, unsurprisingly called JavaServer Pages, was recently written by Hans Bergsten and published by O'Reilly and Associates. I received my copy of this book several days before submitting this column and have been extremely impressed by its depth, clarity and attention to detail.