The software giant's Xbox game titles will be developed into a series of hard/soft, audio and electronic books by Ballantine's Del Rey imprint under an exclusive worldwide agreement just signed by the two companies. Del Rey v-p and editor-in-chief Betsy Mitchell and executive editor Steve Saffel, announcing the new agreement, said three new novels are tentatively planned for release this fall: one is a new Halo title (the first such was done jointly by the two companies last fall), and the others are based on The Unseen and Crimson Skies.