This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Storing items in Session Variables provides many advantages. There's a free online version of the first edition of Core Servlets & JavaServer Pages avaialbe at the website www.coreservlets.com (the link to the pdf is down towards the bottom of the home page). There is a chapter on Session Management which disusses the advantages of session variables over parameters. Keep in mind that that edition of the book is based on the older Servlet & JSP specs (specifically the Servlet v2.1 & 2.2 and JSP 1.1 specs). A lot was added in both the 2.3/1.2 and 2.4/2.0 versions of the specifications. Nonetheless, the basic concepts of session management remain the same, but the tools (i.e. the available classes and methods) have changed.