A slew of recent deals at Ballantine have been devoted to keeping aboard authors who have been doing well for the house. Stephen Frey, who writes financial thrillers, has signed for another four books, all starring a series character still to be introduced: someone, like Frey himself, who runs a big investment firm. Mark Tavani signed the world rights deal with agent Cynthia Manson. In the meantime, Frey's sixth book for Ballantine, The Chairman, will come out in spring 2005.It was editorial director Linda Marrow who signed on the other two house favorites for multibook contract extensions: for Tess Gerritsen, author of medical thrillers starring a Boston medical examiner (made for U.S., Canada and open market with Meg Ruley at Jane Rotrosen) and for romance author Suzanne Brockmann (signed with Steve Axelrod for world rights).