Rory, if you haven't read Ludlum, look at the above. Some of those books were written by other authors after his death.
I thought his best books were written by him in the 70's and 80's. I don't know if they now stand the test of time. I do remember really liking The Matarese Circle, Trevayne, The Osterman Weekend, Holcroft Covenant, and the Bourne books.