This is a fine start to a library - and at a fantastic price.
I envy anybody who gets to start the Hornblower series, in particular, from the beginning. (Naval fiction fans divide themselves into two armed camps: the O'Brianites and the Foresterians. I tend to prefer Forester, but I admire O'Brian too. You'll find that the two have extremely different styles. I think it's generally agreed that Forester is easier to read - especially for anybody who'd new to this sort of thing.)
The four Hornblower titles you're missing are Hornblower During the Crisis, Beat to Quarters (British title: The Happy Return), Ship of the Line, and Flying Colors. Hornblower During the Crisis is a fragment of the novel Forester was working on when he died; the story unfortuately ends just when things are starting to get really interesting. The book is rounded out by a couple of Hornblower short stories. You might also want to keep your eyes open for a copy of The Hornblower Companion. It contains maps, pictures, and Forester's own notes on how he happened to write the novels.
Lever's Young Officer's Sheet Anchor is a classic - tremendously valuable for modelers, especially when it comes to rigging. Just bear in mind that it relates strictly to the British Royal Navy during a specific time period: the late eighteenth and very early nineteenth centuries. (Some editions include additional notes on different practices in the merchant service, and update the material to about the 1840s.) But lots of the basic vocabulary in it (the names of sails, spars, and rigging lines, for instance) is pretty applicable throughout the sailing ship period.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.