I've read Core Servlets and JavaServer Pages(marty hall), which is a good introduction. But if it's servlets your interested in, I would definately recommend Java Servlet Programming(hunter,crawford) - really good!. I think both of these books are listed on javaranch in the bunkhouse.....
The early bird may get the worm, but the second mouse gets the cheese.........<br /> <br />Sun Certified Programmer for Java 2 Platform<br />Sun Certified Web-Component Developer for J2EE Platform
pdf.coreservlets.com has free version of Core Servlets and JSP. While this is a good book to learn JSP and Servlets, as Madhav said it follows old versions. Don;t forget to download servlet 2.3 and JSP 1.1 specifications when you start preparing for the certification.