[Sa'ar Chasm]

I'm re-readed Mr. Midshipman Hornblower while waiting for the library to put Admiral Hornblower in the West Indies back on the shelf. I highly recommend that series. Roddenberry used it as one of the inspirations for Star Trek, and is the source of "Aye aye, sir", "belay that" and all the other nautical gibberish spouted by the characters (although I've yet to see anyone say "Man the scuppers and keelhaul the halliards!" on the show).

Harry Turtledove's novels are good if you like alternate history, not so good if you like well-drawn, believable characters or sparkling dialogue (think Chakotay as a writer). He has a degree in Byzantine history, and writes like a college professor: belabouring the point in a dry, boring voice.

Better alternate history (although with a decidedly military bent) is written by David Drake (eg, The General series written with -->) S. M. Stirling (eg, Island in the Sea of Time, which was ripped off by -->) Eric Flint (1632 has a similar premise as Island, but the Belisarius series written with David Drake is pretty good, even if is does cover similar material to The General).

And, of course, there's always Discworld, or anything by Arthur C. Clarke.