Commodore Hornblower is a novel written in the mid-20th century on Earth.
Jake Sisko had a copy of this novel stored on his PADD in 2372. (DS9: "The Visitor") Later that year, he was reading the same text excerpt again, when he met the muse Onaya. (DS9: "The Muse")
Background
Commodore Hornblower, (or The Commodore, as it is known in Great Britain) was written by C.S. Forester. The novel recounted the adventures of Royal Navy Captain Horatio Hornblower in the Baltic Rim during the latter-portion of the Napoleonic Wars. Hornblower, of course, was the basis of the protagonist of Gene Roddenberry's initial proposal for Star Trek.