Marty,
If you have the contacts and credibility with authors through your other website, I say go for it - you don't necessarily have to get the "author assistant" certification.

IMHO, credibility in this business is built by demonstrating knowledge through networking and endeavors such as your book review site. Certifications from reputable organizations can help, but are definitely not a "must" to get started.

If you have a passion for helping authors and feel like it's something you like to do, I say go for it - put out some feelers to your author contacts and see if you can get some feedback.

Good luck!