Penumbra

Penumbra, the fourth book in the Traces of Treasure Series, continues the adventures of treasure hunter Sam Milton and his girlfriend Smidgeon Toll, who find themselves immersed in the search for a missing man they have never met. As each murky veil of mystery is stripped back, more friends and acquaintances get involved, and the clues multiply.
Their search culminates near a remote desert mountain and involves forces no one could possibly expect. They face deadly adversaries from this world and beyond, and must rely on their loyalty, wits, and bravery, along with a hefty dose of magic, to persevere.