JSTL provides a set of common tag libraries for JSP programmers. This book explains the JSTL and it does it extremely well. In about 200 pages, the book covers what the JSTL is, what to use it for, how to use it, and gives plenty of examples.
The start of the book covers the basics of tag libraries and explains why we need JSTL. Next, the basics of JSTL and the expression language are covered. The one small flaw in the book is that the expression language could have been covered in a bit more detail. The rest of the book covers each of the tags (actions) broken up into the separate libraries. The core, XML, internationalization and formatting, and SQL actions are each given their own chapters. The author doesn't just cover the tags but also provides enough background information to insure that you can understand how the tags are used. For example, in the XML chapter, the author starts by explaining the different technologies around XML and then shows how the XML actions can be used to simplify the task of using XML in your JSPs. The SQL chapter explains why you would never want to use the SQL actions before she discusses the actions themselves. The book ends with a "quick reference" section.
Sue Spielman has a very easy writing style that makes reading her books a pleasure. Her book is short and complete, a very difficult combination to pull off.