In one Honor Harrington novel, the title character is reading a Hornblower book. And, in fact, the whole series had its genesis as a "Hornblower In Space" book; the matching initials are a deliberate Shout-Out and the first book is dedicated to C.S. Forester himself.
One book in the Alan Lewrie series (depicting a much less heroic naval hero) has the main character notice of a tall, thin, and melancholy-looking lieutenant in a threadbare uniform at the Admiralty during the Peace of Amiens—a tacit reference to Hornblower, who was without either a ship or half-pay during that time.
The third Ciaphas Cain book mentions a navy commander named "Horatio Bugler."
Harry Dresden, though he knows how to steer a boat, mentions that he's no Horatio Hornblower.