In the lesson entitled "British Sound Years, Part 3: Collaborators: Hitchcock and Writers - Spotlight on British Writers of the 1930s" Dr. Edwards says:
"Hitchcock will freely adapt the novels to suit his needs as a cinematic storyteller, but feels that the more important the writer or the book, the less lee-way he has to do a freer adaptation; tends to follow major books more faithfully than what he perceives to be minor works."
John Buchan"s "The Thirty-Nine Steps" was a very popular book, especially with soldiers during the First World War. It spawned four Richard Hannay sequels.
The Hitchcock version is radically different from the novel (and much better). In the novel there is no Pamela or Mr. Memory. The "39 steps" are an actual staircase. There is very little in common between the film and the book.
Did Hitchcock consider "The Thirty-Nine Steps" to be a "minor book"? Surprisingly, John Buchan supposedly liked the film very much.