Kolymsky Heights

Kolymsky Heights is a thriller novel by Lionel Davidson.
Plot summary.
A coded message is smuggled out of Russia, a plea for help from a supersecret laboratory deep in the frozen wastes of Siberia. The note is addressed to Johnny Porter, a Canadian Indian of the Gitxsan tribe with a genius for languages and disguises, and reluctantly he is forced to slip across the border on a rescue mission, the consequences of which he little imagines. The detailed picture of life in the Kolyma region and of the native peoples of the Russian Far East (such as the Evenks) and British Columbia (such as the Tsimshian) is impressive.
Critical opinion.
Philip Pullman has said of the novel: "The best thriller I've ever read, and I've read plenty. A solidly researched and bone-chilling adventure in a savage setting, with a superb hero." 
Reviews.
Chosen by Toby Young to discuss on BBC Radio 4's A Good Read with Harriet Gilbert and Sarah Vine