The Dickson McCunn series by John Buchan consists of 3 books, published between 1922 and 1935. The series begins with Huntingtower.
The Leithen Stories series by John Buchan consists of 5 books, published between 1916 and 1941. The series begins with The Power-House.
The Richard Hannay series by John Buchan consists of 6 books, published between 1915 and 1936. The series begins with The Thirty-Nine Steps.