Depends what you are intending to do. LINQ is just a way of dealing with collections/lists of data. It can also assist you with connecting to data stores.
What I am saying is that it is just a language feature, and so the best book to buy is one that deals with a version of your language of choice. They usually introduce LINQ quite well. The best ones are the ones that deal with the technology you want to use, eg, ASP.NET. In this case:
Pro ASP.NET 4 in C# 2010, Apress, Matthew Macdonald, Adam Freeman and others was good enough for me.
As ever, the best way to learn these things is to just start working with them. So if, for example, you wanted to use ASP.NET MVC 2, I would buy Steven Sanderson's book (Pro ASP.NET MVC Framework 2) as well. There is a great example in the first few chapters that you can follow. The book has the added advantage that it teaches you to think in MVC.
Once you get to grips with working with LINQ, you can fine tune your knowledge with:
Programming Microsoft LINQ, Microsoft Press, Paolo Pialorsi and Marco Russo.