"Seamanship In The Age of Sail" by John Harland. Yeah, yeah, that's all well and good but I still don't feel like I can sail a sloop in my spare time. Here's the one, perhaps only, book you've been looking for. Detailed descriptions, complete with illustrations and diagrams, of how tall ships operate, nautical terms, the workings of sail, line and tackle and the language seamen of the era actually used. This is for hardcores only but given how much time our characters spend on a ship it might be the single most important book for realistic storytelling (at least in forum RP). It's not as obtuse as some similar works, while also being far more complete, and it is accessable to the motivated layman reader.
Why, I think we need more references to literature like these! And oh dear, aside from Pirate Latitudes... we haven't been reading. Oops :3