I am interested in a specific author, and there are a lot of her books on PBS.  Is it possible to sort by "poster" to see if anyone has a collection of that author's books?

I don't think so.  I've never seen an option like that here.

The nearest thing I can think of is if you request a posted book (a book on someone's bookshelf), the system lets you go to that person's bookshelf to let you order more books from that same member and you can check then if that person has more books by that author posted.

I don't have BoB, and maybe there are some BoB options that let you do something like that, but I haven't read about it in the forums yet.  ---Thanks to Lita posting below about BoB options, I wasn't aware of this option, very cool.---

If I were you, I'd submit a post in the Book Bazaar section that you were looking for members who have lots of books by that author on their bookshelf.  Then people could pm you and let you know where to look.

---I'd still post in Book Bazaar, because you might catch the attention of someone who has a lot of books by your author who hasn't posted them here yet, or have a chance to get a deal on more than 1 book for each of your credits.---

I go to the Box-O-Books section and search for author.  That will show you which boxers have a lot of that author.  You don't have to be a boxer yourself to search the boxer shelves.

I would post in Book Bazaar and maybe on that genre's page and say what you are looking for and see what happens. Or put them in your signature.  Most likely you will get an offer from someone.  I was looking for books in a Cherry Adair series. Someone had 7 of them and I got them all for 4 credits plus an unpostable book I had on my WL.  They are getting rid of some books and saving some postage. I'm getting a few extra books for my credits.