As of now there are few 2012 books in the market and some more to be released in the coming months but majority is Technet and the Introduction to 2012 ebook which is free. Apart from that try Microsoft Virtual Academy which has a ton of videos for 2012 (also free) In case you hear anything otherwise please share
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If you look in Amazon you'll find some books coming up between Dec-Feb. If you want to start right now you can also look into this: Windows Server 2012 "Early Experts" Challenge