The official documentation can be downloaded from JCP JSR 245 site. The spec consists of two documentations: JSP 2.1 specification and EL specification. Javadocs for the API can also be found from Java EE API.
For users that develope web applications with an IDE, JSP 2.1 is bundled with NetBeans. The integrated IDE provides excellent framework for JSP and web development, with fast edit, deploy, test, and debug cycle.