The xUnit architecture has become the deFacto standard for unit testing tools. There is a great quote from Martin Fowler in which he states:
It's true you could build the core of xUnit in the language of your choice in a short period of time. In fact, since I wrote NUnit 2.0 that is the way that I learn a new programming language.
Over the past few years the xUnit tools and unit testing itself have become more widely used when developing software. I believe this is a very positive step for software development. What has been missing is a collection of what it takes to write really good test code. The book "xUnit Test Patterns" by Gerard Meszaros captures this skilled knowledge and documents it in the form of patterns. In addition there are also sections on how to identify bad smells which can make your test code difficult to maintain. You can get a detailed introduction to the books content at Gerard's web-site here.
I am hoping that this book does for xUnit and test code in general what the Design Patterns book has done for obect-oriented design.