About recommendation
My recommendation is always to not buy a book, but rather to study the Flex Builder help system.
No book can possibly cover the depth of material the help system covers, and it is so well written that in two weeks, two or three hours a day, you will gain a really great perspective on the important aspects of Flex, such as events, containers, controls, states, effects, MXML/ActionScript, etc.
But don't read the whole help system, read the section entitled "Flex Programming Elements" within the major section "Adobe Flex 3 Help" first.
Then read the other sections at the same level of "Flex Programming Elements".
Others may disagree, but if you want to learn Flex slowly, buy a book or two. If you want to truely know Flex, study the help system.
If this post answers your question or helps, please mark it as such