The Eiger Sanction, Trevanian. First Edition.

I love the vivid cover on this one. Trevanian was the pen name for Rodney William Whitaker (1931-2005), who was an American film scholar and writer. He wrote in several different genres, achieved best-seller status and also wrote under numerous under pen names, including Nicholas Seare, Beñat Le Cagot and Edoard Moran. He was an intensely private person, avoiding publicity for much of his life, and his real name was kept a secret until 1998 when a Texan reporter discovered and published it. He mainly wrote thrillers, with his first two books being smart little spoofs on Bond novels, but it remains an open question whether he was being playful with the genre or expanding its limits. Per the New York Times, "Trevanian was the only writer of airport paperbacks to be compared to Zola, Ian Fleming, Poe and Chaucer."

The Eiger Sanction was Trevanian's first novel, published in 1972. The plot involves a classical art professor and collector who doubles as an assassin and is coerced out of retirement to avenge the murder of a friend. The novel was adapted into a film starring Clint Eastwood in 1975. This is the first edition:

Trailer for the 1975 film starring Clint Eastwood:

Rodney William Whitaker (Trevanian):

More info here: