From Halopedia, the Halo wiki
Del Rey Books is a branch of Ballantine Books, which is owned by Random House. It is a separate imprint established in 1977 under the editorship of author Lester del Rey and his wife Judy-Lynn del Rey. It specializes mainly in science fiction and fantasy books.
Del Rey was responsible for the publishing of Halo: The Fall of Reach and Halo: First Strike by Eric Nylund, and Halo: The Flood by William C. Dietz. Afterward, publisher Tor Books took over the publication of the next several Halo novels.