A good book for my dad…

Hi, answer person! I hope I am asking an appropriate question…
My dad loved reading the Patrick O’Brien nautical series (Master and Commander, etc). He was also addicted to C.S. Forester’s Horatio Hornblower series for a time. Another favorite was Colleen McCollough’s Ceasar series. Unfortunately, he’s completed all three series, and my never-fail holiday gift idea of the past three years is now defunct! Can you suggest another series/book he might enjoy reading?
Thanks!

ANSWER PERSON RESPONDS: Answer Person suspects your father may have already read the Alexander Kent books; if not, go for it. Other possibilities: Dewey Lambdin’s Alan Lewrie tales, Edward Cline Sparrowhawk series, and the works of James Nelson.