We have quite a few books about Flash development. Probably the best ones for getting started are the Missing Manuals and the Learning books. Depending on what version of Flash you'll be using to develop your site, there are different editions of these books.
If you're using Flash CS4, some good books are Learning Flash CS4 and Flash CS4: The Missing Manual.
If you're using Flash CS5, check out Flash CS5: The Missing Manual and Learning ActionScript 3.0, Second Edition.
If you're using Flash CS5.5, we have a new Missing Manual coming out later this month, Flash CS5.5: The Missing Manual.
I hope this helps. Have a great day!