One of my favourites is WROX's Professional JSP 2nd Edition
It's a bit big, and maybe pricey, and also, it's not a 'servlets' book like you asked for. But, the 2nd edition does a very good job in the first three chapters of explaining the evolution of backend processing, and what servlets do and how to code them, and *then* it takes you to JSP.
So it's a great book if you are involved with writing both servlet and JSP.
The 'servlet by o'reilly' referred to by someone else is properly called O'reilly's Java Servlet Programming 2nd Edition. This is also an excellent book and I have a copy of it as well. Very thorough treatment of *just* servlets.