Halloween Kills: The Official Movie Novelization will be published in mass market paperback, e-book, and audiobook on October 19 via Titan Books. The film opens on October 15, so no need to worry about spoilers.
The 320-page book is written by Tim Waggoner (Alien: Prototype, Supernatural: Mythmaker), based on the script by David Gordon Green, Danny McBride, and Scott Teems. The synopsis is below.