Charlie Calvert and Dinesh Kulkarni have just released their new book, Essential LINQ. 

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LINQ was introduced with C# 3.0 and VB 9.0 (shipped with VS 2008) and stands for Language Integrated Query.  The technology allows you to use SQL like syntax right inside your code.  For example:

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This simple C# expression declares an integer List object with four elements and allows me to query out all values greater than 12.  The resulting variable q then has the numbers 15 and 20 in it.  LINQ is very powerful in that the objects you query can include any data type represented by the CLR.  That means you could be querying inbox mail messages from Outlook, SQL data, SharePoint lists, etc.

Essential LINQ walks you through everything you need to know to take advantage of LINQ starting from the basics and then working all the way up to transacted database applications.  Highly recommended reading!