You can download a free copy of the PDF version of the book Getting Started with Flex 3 by taking either of two quizzes available at SitePoint. Details regarding the free PDF are available here. The gist of it is that you can download the PDF without registration by simply taking a very short quiz at the end of either of two Flex-related articles by Jack Herrington. I believe that this promotion runs through 22 September 2008.
The first article is called Build a Web 2.0 Voting Widget with Flex, Part 1 and the second article is called Building a Web 2.0 Voting Widget with Flex, Part 2. Both articles provide nice background and information on using Flex and the quizzes are short and painless and help to ascertain your knowledge of Flex.
While at the SitePoint site, other Flex/AIR-related resources you may want to check out include An Absolute Beginner's Tutorial of Flex 3, Walk on AIR: Create a To-do List in Five Minutes, and Create a 3D Product Viewer in Flex 3.