Marketing is two things for an author: brand and buzz.
Brand: This is where you decide as an author what to write so that you stand out in the market.
Jenny Crusie is the queen of real-life snark.
Tom Clancy is spy action.
John Grisham is legal thrillers.
I am Asian Christian chick lit.
Robin Caroll is Cajun/Southern romantic suspense.
Cheryl Wyatt is military action romances.
Deeanne Gist is slightly edgy historical romances.
Brandilyn Collins is small town suspense.
Colleen Coble is small town romantic suspense with animals.
Donna Fleisher is angsty military women's fiction.
This is the hardest part for writers to figure out. You have to actually tweak your writing so that your books will stand out from all the other pitches editors hear.
If you have a solid brand, editors are much happier to read your stuff because you're classified in a certain file in their heads. They can sell you easier to the VP of Sales and VP of Marketing because you're branded. An editor may love you but they're dictated to by Sales and Marketing, which is why so much good writing is not contracted.
It's not just looking at what you've written, but also looking at what you're willing to write, or willing to revise. Branding isn't just looking at your completed manuscripts and trying to find a common theme--it's looking at your own desires in writing, what you're willing to write about, how you can tweak your writing so that it's more marketable.