Finkelstein and Leete have written the perfect introduction and reference for Flash.
I recently started using Flash because one of my clients' legacy websites had a couple of Flash animations that needed to be updated. I stumbled around with Macromedia's documentation and a couple of online websites, but somehow Flash kept doing things I didn't expect.
Finally, I found this book and read it cover to cover. "Oh, of course!" I kept saying as I read. "There's a neat technique for layering. That's how to reduce the file size. That's how to make it run faster. That's how to get the sound to synchronize with the motion." On and on. Macromedia should just ship this book with their software. Plus the book comes with a CD-ROM packed with useful stuff.
My client is thrilled with my new versions of his Flash animations.
I want to especially commend the authors on Chapters 12 and 13, "Putting it all together" and "Publishing your Flash files." This is where I found that information that showed me how to put out a clean professional final product.
I've gotten so into Flash that I rewrote a complete interactive game in Flash. It used to be in Java, was 500,000 bytes (not to mention the 2Mb Java library), and only seemed to run correctly on a handful of machines. In Flash, the whole thing is under 100,000 bytes and runs EVERYWHERE no problem.
I couldn't have done that rewrite without this book.
When you're done reading this book, I also recommend "50 Fast Flash MX Techniques" by the same authors, to take your animations to "WOW!"