You're introduced to Subversion somewhere you are working. You use it more and more, then find that you have become the resident expert in your team. How did you get there? Well, you probably started with "Pragmatic Version Control Using Subversion", picking up good advice on practices, a gentle reference for common operations, and a very easy to read tutorial. After that, you probably moved onto "Version Control with Subversion" which is more reference than tutorial, containing more detail on those bits that most people don't encounter very often (e.g. how to flush user authentication information). That probably saw you through a number of difficult questions from colleagues. But after that, when you need to do REALLY nitty-gritty stuff with your repository where do you go? So far, my answer is "Practical Subversion" by Daniel Berlin and Garrett Rooney.
As an example, if you want to write code that talks to the Subversion APIs directly rather than calling command-line operations and parsing the results, then this is the book for you. It's never going to be a complete reference on that particular subject, but it gives you enough information to get you started and points you to the place to find the complete API reference.
It's also good on administration stuff too, as well as giving information about a number of useful third-party tools for both administrators and developers that other books don't mention.
So once you've done "Pragmatic Version Control Using Subversion" and "Version Control with Subversion", then this should be next on your list. Go in that order though - this is really for power-users of Subversion (developers or administrators), it is not an introduction for people who just need to get the occasional document out of their company's repository.