Welcome to MobileRead, hish747!
I'm working from memory here, because I had to send the eval 505 they lent me back to them, but I think when you go into the authors listing on the 505 it goes to the second page of the listing, if you hit the "back/previous page" button it should take you to a grouped alphabetical listing on page 1 (i.e. A through D, E through G, or whatever). That will let you drill down some, though I'm not sure how close that is to what you're wanting.
If I'm misremembering, I'm sure one of our 505 owning brethren will happen by and correct me.
The authors are sorted by the way the names are entered in the book-file's meta data (this is the same for the 500 and the 505), if the name is entered as FirstName LastName it'll sort by FirstName, so obviously for it to sort by LastName, the meta data entry would need to be LastName, FirstName.
For non-encrypted ebooks, that's easily enough edited with a number of tools. For Sony Store books ... not so much. I don't think libprs500 will even adjust those, but I haven't tried it, so ....
I'm glad you've found a device that works well for what you need to do. That's the important bit, after all.
