The Captain's Messenger — Anne Holman

When Clare's wealthy, widowed aunt decides to remarry she expects Clare to act as a go-between for herself and Captain Peter Chatham. Peter has no intention of marrying, and to get himself off the hook, introduces the aunt to another officer. While Clare is running messages, a mysterious man who has been attacking women in Bath, kidnaps her. Peter rescues her and realises he might like to marry after all.