There are 2 books in the Nicholai Hel Series series. The newest book is Satori which came out in 2011.

1
Shibumi

Book cover of Shibumi
A classic spy novel from the bestselling author, Trevanian, about a westerner raised in Japan who becomes one of the world's most accomplished assassins.

Nicholai Hel is the world’s most wanted man. Born in Shanghai during the chaos of World War I, he is the son of an aristocratic Russian…
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2
Satori

Book cover of Satori
Nicholai Hel--genius, mystic, and the perfect, formidable assassin--was first introduced to readers in Shibumi, the classic #1 bestseller by master storyteller Trevanian. Now, critically-acclaimed author Don Winslow continues Hel's story for the first time in this all-new, blockbuster thriller. It is the fall of 1951 and the Korean War is…