As Adobe released Flex 4 SDK few days back with flash builder, it seems like a good time to make chart of all the good Books coming out. Some books are published and some are on the way. This is a little work to gather all the books information for flex 4 here so that it will save some time.
1: AdvancED Flex 4
2: Flex 4 Fun
3: Flex 4 in Action
4: Hello! Flex 4
5: The Essential Guide to Flex 4
6: Flex 4 Cookbook
7: Flash Builder 4 and Flex 4 Bible
8: Effortless Flex 4 Development
9: Adobe Flex 4: Training from the Source
10: Flex 4 Solutions: Essential Techniques for Flex Developers
11. Learning Flex 4
12. Adobe Flex 4: Training from the Source vol 2
13. Enterprise Development with flex
Besides these books Adobe's documentation are really good to follow.
Again in the future other books may come. If I have missed any book please let me know, so that I can add to my chart.