book to explain old rigs?

[Brion Toss]

Oy,
It must be lovely to embark, as it were, upon the discovery of this grand vernacular. I'll second Brian's suggestion, and add, for starters, all the Aubrey/Maturin books. The author uses Maturin's ongoing education to illuminate the reader in the way of the ship, and the stories show the ship in use. There are also some splendid companion volumes to the series available, for everything from cuisine to charts.
But of course nothing can compare to the real thing, so be sure to visit the nearest square-rigger as soon as you can, for as long as you can. At first it'll just look like a mass of sticks, but in time will resolve itself into the elegant system that it is, with all of those now-puzzling parts easy and obvious, but no less lovely.
Fair leads,
Brion Toss