Barbara [ http://www.anobii.com/anbra6/books ] makes it sound as if this masterpiece of modern spy literature was written with EFL, teaching of English as Foreign Language in mind [ http://en.wikipedia.org/wiki/English_as_a_foreign_or_second_language ].
Not so – and, as for it being "simple" (=as in simplistic) and "linear" – Barbara doesn't know the meaning of those words. While nominally about the business of spying, the main theme of this book is the process of achieving maturity, the essence, unattainability and price of life-time friendships. Long after all other John Le Carré's works have been forgotten [hope not], this one, and his "Little Drummer Girl"[*] will live on as type-examples of best XXth century biographico-epical-mashup fiction.
[*] "The best recent novel about terrorism was published in 1983, and its author was John le Carré, better known at the time as a crafter of cautionary tales about the intelligence battles of the cold war."