The following is a list of venues for purchasing the ebooks and audiobooks and internet resources about the books themselves or their historical or geographical context. We'd love to add additional sources; just post them to this thread and they'll be added to this post.
eBooks and Audiobooks:
Amazon US
Amazon CA
Amazon UK
Audible (Patrick Tull narration)
Audible UK (Ric Jerrom narration)
Kobo US (couponable!)
Kobo UK
Kobo CA
OverDrive eBook and audio (Simon Vance narration)
Resources:
Musical Evenings with the Captain: Music from the Aubrey-Maturin Novels
Musical Evenings in the Captain's Cabin
The Patrick O'Brian Mapping Project: The Voyages of Jack Aubrey and Stephen Maturin
A Sea of Words: A Lexicon and Companion to the Complete Seafaring Tales of Patrick O'Brian
Reviews:
Jo Walton's re-read !! SPOILERS !! Avoid until AFTER you've read the novel