Book of Souls - Cooper, Glenn

Book of Souls is the follow up to Secret of the Seventh Son. You should read them in order. In this book retired agent Piper is sucked back into the secret of the ancient library of death stored in area 51. There is one book missing from the library, and when it shows up at an auction, agent Piper is convinced to follow up on the book's history.  It's an amazing history.  Plenty of action and suspense and mysteries to solve.  Very highly recommended.