Beginning Writer's Answer Book by Jane Friedman
Should I hire an outside publicist for my book?
If you can afford it, yes. A 2006 study found that authors who had outside help sold an average of 10,000 copies for their first book and earned royalties averaging $55,000, while writers who depended solely on the efforts of their publishers sold 4,500 copies and saw royalties of $25,000. The majority of authors who hired an outside firm said they would do so again.
A publisher will not be offended if you hire your own publicist (or marketing firm), who will work in tandem with the publisher to make sure there isn't duplication of effort.