Pretty good book,
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This review is from: Microsoft SQL Server 2012 High Performance T-SQL Using Window Functions (Paperback)
The book is split in two (in my opinion): the first part explains everything about the window functions implementation in SQL Server 2012 (and also what is missing still from the standard); the second part is about optimizing window functions.
However, it's one of those books that is so well written, that you just read it once, grasp the concepts, and never look back to it. I got the paperback, but now after reading it I wish I had bought the Kindle version.
As a SQL Server Professional, I rate it 4 stars, mainly because most of the stuff in the first part can be found on the Internet (up to a certain extent) or in other books Itzik wrote, but definitely the second part (Optimizations) it's pure Itzik magic that you won't find easily on Google.
Plenty of example code, explanations and lessons to be learnt in this book. Also some shockers (eg. when a RBAR cursor is faster than a set based approach), and even more shockers (I'll let you find out).
Must buy (on Kindle).