The novels by Christopher Golden (
The Lost Army,
The Bones of Giants, and
The Dragon Pool) are canon, but only in a loose sense. So the events of the novel are canon, but the small details, like say the history of Hellboy's hairstyle, are not.
There's also a few short stories that are canon:
- "The Nuckelavee" (in Hellboy: Odd Jobs)
- "The Promised Smile" (in Hellboy: An Assortment of Horrors)
- "The Other Side of Summer" (in Hellboy: An Assortment of Horrors)
Christopher Golden has spoken about adapting some of his novels to comics in future. Not fully, but bits of them set in new stories, similar to the way the
Baltimore novel was adapted to comics. He did stress that there was no plan to do this though, it's just something he'd like to do someday.