These days, none of the books cover anything I don't already know. Just before the old Wrox became whatever it is today, there was a flood of specialty books like C# Threading handbook, Professional ASP.NET Performance, Professional .NET Network Programming and C# Text Manipulation, among others. I bought them all.
These days, it's not that I know everything (I should know a lot more about XML and Web services, for example, but I already have those books and they're boring), but the really detailed stuff isn't out there anymore. This isn't that surprising anymore, because I know some publishers won't even touch a book unless it can sell 10,000 copies. That's a shame, because while the Internet does have a lot of fabulous resources, it's still too much of a mess at times to get a clear vision of how to do accomplish something. Books make it so much easier.
I hope and pray that my book meets that five-digit sales figure. It's one of the few really intermediate books out there, in my opinion, and I think it's a market that has been neglected, or at the very least books haven't been positioned for that market. I've got eight chapters in the can, nine to go.
Incidentally, I've bought lots of non-computer books lately. Rebel Without a Crew, Masters of Doom and now My Life, are the most recent. Weird that they're all biographies or autobiographies. The last computer books I bought were Test-Driven Development in Microsoft.NET (remarkably useless for anyone that has heard even a little about TDD) and Managed DirectX9 Kick Start (thin on theory, as intended, but an outstanding introduction to the subject).