The Book of Skulls by Robert Silverberg
Paperback. Condition: Good+/Very Good- 1972. Signet New American Library. 1st Edition, 1st Printing.
Four students discover a manuscript, The Book of Skulls, which reveals the existence of a sect, now living in the Arizona desert, whose members can offer immortality to those who can complete its initiation rite. To their surprise, they discover that the sect exists, and is willing to accept them as acolytes. But for each group of four who enter the rite, two must die in order for the others to succeed.