From what I have seen and read, Powershell isn't a replacement for Windows Script Host, it's just a new, more powerful shell for the OS. There are things that you can do with VBScript that you can't do with Powershell, and vice versa. Powershell requires the .NET Framework, and not all systems will have that installed, so your VBScripts will still work fine, even on Longhorn and Vista.
The book that I bought was the "Microsoft Windows 2000 Scripting Guide". Everything in it is also applicable to Windows XP and 2003. This is a very good book. It's great for people new to scripting and is a great reference for people who have been writing scripts for years.
There is also a book called "Advanced VBScript for Microsoft Windows Administrators" that from the description at Amazon looks similar, but based on looking at the table of contents it probably has much more in-depth info, especially where it comes to interacting with applicatons like SQL and Exchange. I may pick this one up, I didn't know about it until just now.
At any rate, both books are from Microsoft Press.