When you're a big-name author (as Kay Hooper apparently is) you get to publish work that might not otherwise see the light of print. Lady Thief is Hooper's very first novel: she's known for contemporary romances, but this is a Regency romance, and one that owes more to the swashbuckling tradition than to Austen or Heyer.
Jennifer Courtenay is set on avenging the death of her father, murdered by a mysterious visitor while investigating treason and espionage during the Napoleonic Wars. In order to find the talisman ring that was taken from him by the murderer, she dresses up as a highwayman, assumes the name 'The Cat', and robs innocent noblemen in their coaches. She's a dab hand with a sword, and she has the help of Jason (a genuine highwayman, middle-aged but devoted to her). There are some issues with her step-father; her sister's ill-considered romance with a penniless lordling; and Jenny's strong, but irrational, attraction to the Duke of Spencer. And there are some more testing issues, such as anachronistic speech, the sort of plot that I can only describe as 'just in time' (with some elements that seem unlikely, to say the least), and some rather two-dimensional characterisation.
It's easy to tell, comparing this and 'Masquerade' -- a novella written rather later in Hooper's career, which deals with the hackneyed 'seeking refuge after failure of vehicle in snowstorm' plot, and manages it with a modicum of grace and style, and some interesting backstory -- that Lady Thief was Hooper's very first novel. It's not especially well-written and there are some problems with plot and setting.