If you are new to performance and load testing, you need two things: knowledge of testing practices and knowledge of testing tools. .NET Performance Testing and Optimization: The Complete Guide by Paul Glavich and Chris Farrell provides both of these.

.NET Performance Testing and Optimization provides a solid overview of good testing practices, as well as practical information on how to use various commercial testing tools that are currently available.

The first three chapters cover principles of testing, such as which metrics you can measure, what information each metric provides, and when to care about each metric.

Glavich and Farrell describe the different types of tests and the goals of each. Performance Testing, for example, measures the speed at which an application responds to user input and events. Load Testing measures the number of concurrent users an application can reasonably support. Although they are often confused, the goals are different and the important metrics for each are different.

The rest of the book describes in detail various types of tests you can perform. Glavich and Farrell even provide step-by-step instructions to perform these tests with popular commercial products.

For those new to performance testing, for those needing a refresher on performance testing, or for those who want to know how to use commercial products to perform performance testing, .NET Performance Testing and Optimization  is a good source of information.