There are 2 books in the The Descent series. The newest book is Deeper which came out in 2007.

1
The Descent

Book cover of The Descent
In a remote Himalayan cave a group of New Age tourists come across the mummified corpse of an RAF flyer. Before they can investigate the mystery, all except the guide are ritually massacred. In Bosnia, an American Air Cavalry patrol investigates a disturbance at a mass grave site and its…
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2
Deeper

Book cover of Deeper
Widely and enthusiastically acclaimed, The Descent marked the debut of a powerful imagination and earned Jeff Long comparisons to Stephen King. Now, after three more suspenseful blockbusters, he returns to the captivating creatures he conjured in that first incredible novel, unleashing a story that will hook his fans and never…