We can now build Web Services and WCF Services based on SOA principles.
WCF Services provide Interoperability between Web Services and any other technology like PHP, Java.
WCF can be hosted on IIS and have a whole lot of features like Instance and Concurrency Management.
WCF has a short Learning curve if learned from right books.
To Begin with SOA start reading SOA Books by Thomas Erl.
The SOA Concepts book is the best place to begin to Learn All about Services.
Then begin with Learning Essential WCF 3.5.
Service-Oriented Architecture: Concepts, Technology, and Design, 1e By ERL
Essential Windows Communication Foundation (WCF): For .NET Framework 3.5 (Microsoft Windows Development Series) - by Steve Resnick , Richard Crane
Windows Communication Foundation 4 Step by Step - by John Sharp