(I recently did a podcast on the topic of female highwaymen in history and literature, and the motif in modern lesbian romance. This is one of several reviews resulting from my reading for that podcast.)
The Locket and the Flintlock has a solid historic romance concept: the carriage bearing Lucia Foxe, her father, and her sister is accosted on the road by a gang of highwaymen and they are robbed of their valuables, including the locket that Lucia’s dead mother left to her and which she loudly protests the loss of. (This is, by the way, lesbian highwaywoman romance standard plot point A.) Alternating points of view between Lucia and the leader of the highwaymen, Len Hawkins, leave us in no doubt of the gender of the latter and that she will be the love interest. A reference to the poetry of Byron and to Lucia’s brother being off in the Peninsular Wars appear to narrow the setting down solidly to ca. 1812-14 or so. The Foxes are members of the rural gentry and Lucia is starting to age out of expectations for an advantageous marriage. The set-up is perfect for her to be swept off her feet by a dashing highwaywoman with a heart of gold whose philanthropic interests extend to supporting the anti-industrial actions of the Luddites.
Unfortunately the core of the story is obscured by the prose style, including overly detailed descriptions of the setting, and the characters repetitiously examining their every emotion, regret, second thought, and aspiration. Beyond that, the writing is solidly workmanlike, other than a tendency for the characters to explain their actions to the reader rather than to experience them.
There are serious plausibility issues with the plot and setting. All the major characters are given to impulsive actions that should long since have proven fatal (especially to highwaymen). Two examples will suffice. Scant days after robbing the Foxes’ carriage, the highwaymen just happen to ride past Foxe Hall at a close enough distance that Lucia is able to recognize their faces (at night, in the dark) from her bedroom window. And having done so, Lucia sneaks out of the manor in the middle of the night, evidently in her nightgown (though with a cloak), and rides her horse bareback after the highwaymen to demand the return of her locket.
Plot holes and world-building holes abound, with the geography of the neighborhood being conveniently elastic depending on whether locations need to be nearby or completely unfamiliar. The author has done her research on many aspects of the historic setting (in particular the Luddite movement) but the presentation of the economics and logistics of early 19th century rural English society left me scratching my head. (There is a startling lack of servants at crucial points, and somehow the household and stable chores of maintaining a robbers’ hideaway don’t involve anyone actually doing domestic labor.)
That said, if you're forgiving regarding plausibility in your historic setting, and you’re willing to overlook the protagonists' suicidal impulsivity in exchange for lots of angsty self-examination and a few hot sex scenes, there’s no reason why you shouldn’t enjoy this book.