Introducing Microsoft LINQ is now a free e-Book available from MS Press. You can download it from here. Microsoft is promoting LINQ hard so as to make it even more popular.
If you have taken a look at CodeGear/Capable Enterprise Core Objects (ECO), you know that UML models + OCL is more abstract and flexible for larger projects, but LINQ is powerful enough for small projects.
I really like the book and LINQ, but the most critical issue that prevents me from using it in real projects is that LINQ requires .NET 3.5 Framework. This redistributable is huge (several hundreds of MB) so I doubt my clients were happy to install it. If Microsoft considers adding those bits to Windows Vista SP1 and Windows XP SP3, OK I will get addicted to LINQ soon.