I've been talking with several authors about an SPS development book but we don't have anything to announce yet.
We are working on a book for the successor to the current version of portal development though. ASP.NET 2.0 Web Parts will be an important part of the future for ASP.NET developers and SharePoint developers. There's not a lot more information to share at the moment but you can look at:
which will publish when ASP.NET 2.0 is available. (Amazon has the publisher listed wrong - this is Wrox, not HungryMinds, so I'm getting that corrected.)
Senior Acquisitions Editor
Wiley Technology Publishing
Jim's Book of the week: Beginning CSS: Cascading Style Sheets for Web Design