If you are interested in a more detailed study of servlets, download and read the Java Servlet 2.4 Specification from the java.sun.com website. Although its title is “Specification,” it actually contains some good explanations of the details of servlets.

After you master the basics here, you can go on to get deeper knowledge from either Hall and Brown's Core Servlets and JavaServer Pages, 2d ed, 2003, Prentice Hall PTR, (make sure you get the latest edition) or Servlets and JSP: The J2EE Web Tier by Jayson Falkner, Kevin R. Jones, Addison-Wesley, 2003.

If you want to know what kind of web server a site is running, or you want to see the market share of different web servers, look at www.netcraft.com. Finally, the website www.servlets.com is a great resource for programmers writing servlets and JSP code.