He was an extremely eclectic writer (originally from NY and then Texas, incidentally, where he'd been a professor of English, Film Arts, and Creative Writing) who moved to the Basque part of France when he was sick and tired of U.S. culture once and for all. (You'd have to read his books to understand why the Basque country of all places.) "Shibumi" is a late 1970s spoof on the spy literature that used to be in fashion then, with an assassin as its central character, but it's also (like most of Trevanian's books, on some level of another) a vicious take down of late 20th century Western culture. (Trevanian's [anti-]hero is a Westerner, but, as the book's title implies, he was brought up in Japan and lives by Japanese values.)
It's enormous fun, as long as you take it with a major grain of salt. The worst disservice you can do to Trevanian is not knowing when to take him literally at his word and when ... rather not.
Trevanian loves to make fun of the usual literary tropes. It's a blast! As Themis stated abobe, don't take him too seriously. But I love his writing. I "discovered" Trevanian more or less at the same time I started reading Ian Fleming.
For more info:
http://www.trevanian.com/
https://en.wikipedia.org/wiki/Trevanian