Summarising briefly: Aubrey takes command of a ragtag group of ships in order to dislodge the French occupiers of Mauritius and La Réunion. Complicated maneuvering ensues, leading to a resounding victory which -- as usual -- Aubrey gets no credit for. It's all told with O'Brian's usual style and wit; recommended.
(I can't help but note that sufficiently detailed historical fiction is indistinguishable from SF.)
ISBN 000649918X. Search for this book on Wikipedia or isbn.nu.