Most of the SQL books I own are related to Administration and specific SQL technologies (such as Reporting Services, Clustering etc). I generally wouldn't pick up a book on troubleshooting, most of the time I can find the information I need on the Internet (I like to think of myself as a fairly good Google-r). However I saw the book Professional SQL Server 2008 Internals and Troubleshooting being promoted around the SQL community, so I checked it out and thought it may actually be a good read. I figured that even if it ended up a paperweight on my desk, I could at least claim the expense of the book as a tax deduction!
Well let me say that this is probably the best money I have spent on a book in a very long time. I only wish I had read it sooner.
The chapter I loved the most was the one on locking and latches. At my previous role we had nothing but trouble with locking and latches. I like to think I did my best at troubleshooting the issues, but there was a point where I was struggling with working out the root cause of the problem. I searched furtively for any information online regarding locks and latches, but there wasn't anything out there that I found useful, or informative enough. This chapter explained everything I wanted to know in detail about how locks and latches work. I have since moved on to a new role where there aren't these sorts of issues, but it is handy to have the knowledge there so when I do run into this sort of problem again I can understand what SQL Server is doing and maybe have a better chance of fixing it!
The other topics covered in this book are also very informative and detailed. I haven't come across any other content before that gives this much information about the underlying components and how the internals of SQL server work. It also has many chapters dedicated to various (free!) tools you can use to help troubleshoot SQL Server.
I don't really know what else to say about this book, apart from I highly recommend you go out and get a copy right this instant. If you have ever been interested in knowing what goes on "under the covers" of SQL Server, how to diagnose problems and actually understand what is going on, then this book is for you. Arming yourself with this knowledge will certainly help when you run into problems and need to do troubleshooting on your SQL Server, rather than try search the Internet for information that just isn't out there.
You can find the details of the book and links to buy it at its website: http://sqlservertroubleshooting.com/