This contains information that Jack and Stephen might know or could conceivably learn about their time. So, science, sounds, RN ranks and ships, politics and people of the era all fit here.
Dr Maturin's Natural History by Wanda Finch.
H.M.S. Surprise (Wayback Machine).
Maturin's Medicine: A medical companion to the canon. Compiled and maintained by Kerry Webb inter alia.
Ranks and Prize Money by Bruce Trinque.
Sounds of the period.
Stephen Maturin - Biographical Notes by Liz Bezera.
The Ships of Jack Aubrey: Informative pages on ships in the Canon by Bruce Trinque.
The Shipwreck of Sir Cloudesley Shovell transcribed by Henry Buurman for the Gunroom.
Three Nautical Lexicons transcribed by Chris Morrison.
Voyages of Jack Aubrey and Stephen Maturin: The Patrick O'Brian Mapping Project assembled by Tom Horn.