In fiddling around with stuff out of a cache to compile the Amazon books-only reviewer top list[1] I noticed that their author ID numbers actually use the same format and database as products. The product ones are known as ASINs, those 10 letter/digit things you see in the product links and such, and uniquely identify all products sold by Amazon. They usually start with B00 unless it's publication that has an ISBN in which case it's the ISBN, though perhaps that's because they haven't hit B01 yet or something.. Customers have another ID, (a customer Id) that starts with A and a few digits - it's usually 14 characters but sometimes 15 and, very seldom, much longer (no idea why). Customer reviews have yet another, starting with R. There's a few more types for various things (wish/favorite lists, follows, virtual server instances, so on..). Authors are given an "Element ID" (I've very seldom seen it mentioned at all) which is what is used for those "See the other books by this author", "See this authors bio" links, but it doesn't have a separate format or, it apears, database.
You can't add them to your cart or One Click buy them (already checked, sorry), but you can go to where their product review page would usually be. It'll be fairly empty, but at least at the moment, you can leave reviews there (and say those reviews were helpful, comment on them, permalink reviews.. anything you usually can on a product review page). Here's an author product page with one review (the one I wrote to see if it'd actually work): Kimi Florez product review page
How do you get to this place for other authors? Get the link to their Author page on Amazon by searching or their name links to it on any book page (right-click -> Copy Link Address to copy it) and use this handy tool:
Or you can copy the ASIN-like code from the authors page and paste it in the correct place in a product page yourself if you want.