Now look, I know I could get into serious trouble here, but I find O'Brien tedious and Hornblower to be utter rubbish. There I've said it. Quite cathartic really. I'm too far away for any of you to find me and keel haul me so I'll thumb my nose at general consensus and be done with it. ;-)
I'd like to second however those who suggested the likes of Moitessier , Villiers, Adlard Coles and Hal Roth.
Possibly a bit too English for many Americans but J D Sleightholme's books are always good for a laugh and can be very educational.
Under the heading of "How could they not have been mentioned before ?" would have to be anything by Eric and Susan Hiscock and if you can still get them Irving and Electra Johnson's books particularly the voyages of Yankee (one , two and three) .
Also worth a read is Don Holm's "The Circumnavigators". Again I'm not sure if it is still in print but it is available as an E-Book free of charge, as is Slocum and Jack London's "Voyage of the Snark." If anyone wishes to find them please PM me and I'll send you the URLs.
Somewhat left of centre and if you havn't read him you should, is the English author now domiciled in the USA , Jonathan Raban. On a nautical theme there is 'Coasting' , 'Passage to Juneau' and 'Old Glory'. Old Glory is a trip down the Mississippi in a tinny so not really sailing but the other two are about voyages under sail. In Passage to Juneau, Raban tells how he decided to buy his current boat when he saw she had some 16 lineal metres of bookshelves. I like his attitude. Raban may be a little bit left wing politically for some of you but he's by no means Joseph Stalin reincarnate, just a damn fine read.