Building upon my earlier posts: -
We have this: -
The WebSphere Application Server Performance Cookbook covers performance tuning for WebSphere Application Server (WAS), although there is also a very strong focus on Java, Operating Systems, and theory which can be applied to other products and environments.
This is amazingly useful, partly because it's available as a PDF as well an online HTML reference.
Chapters include: -
etc.
Definitely worth adding to the reading list ...