Unfortunately I found this to be a stuffy contemporary literary novel in historical clothing, with none of the brio of Austen's own style and little insight to contribute about the characters or story of Pride and Prejudice.
There's not much logic in how the plot of this book fits in with the above-stairs developments of Pride and Prejudice. The action of Longbourn doesn't consist of previously unseen repercussions of those familiar events, nor does it posit any new motives or influences that provide alternate explanations for them. At times, it feels as though Baker's characters are waiting for something to happen in P&P, which only makes sense if you see the plot of P&P as necessary or guaranteed--which you can't, because the characters in that book are frequently surprised by news, choices, and revelations of the past. Wickham appears here as a scoundrel, which we already knew, and the author seems very pleased with her insight that Mr. Collins and Mary would have made a good match--something that I think every reader of P&P perceives and a luscious bit of permanently unresolved dramatic irony on Austen's part. Baker adds backstory for a few major characters that can't feel consequential because it's entirely unmanifested in P&P. Unlike Rosencrantz and Guildenstern Are Dead which you could superficially say interacts with Hamlet the same way this wants to with P&P, this book doesn't provide us with any cleverly interlocking alternate explanations, new plot twists, thematic extension, or characterization.
Baker tries too hard to convince us of her commitment to gritty historical realism; her frequent mentions of chamberpots, menstruation, mud, etc. are cloying and to me seem to lack historical logic. Would someone of that period spend so much time thinking about the dirtier aspects of existence, or would they view them as given, as background? I'd much rather read something that also acknowledges the beauty of historical times, like, for example, the passage in Doomsday Book about the snowy medieval Christmas. Notably, Doomsday Book has plenty of gross scenes--the point is that it has both, whereas this book perhaps unconsciously betrays a modern viewpoint by dealing mostly in grime and unpleasantness. Similarly, there are moments where the protagonist, Sarah, acts in bold or independent ways that seem implausible for a character of her station in that period and unjustified by her personality. They seem like things a modern young woman would do, so they only work if you are putting yourself into the story in her shoes.
I've complained a lot about this novel's relationship to P&P. Can it be considered successful as a freestanding novel? Yes, somewhat, it's just that then it is a novel in a genre that I almost never enjoy. Several reviewers have said that Baker writes like Austen, which I don't think is remotely true. Austen wrote a brisk drama/comedy of manners; this is a ponderous romance. Austen was matter-of-fact and sometimes pert; this is self-serious and tries to assign mystic import to even prosaic life events. Austen requires you to read between the lines of straightforward-seeming dialogue and descriptions to discover a character's motives or mindset. Baker writes paragraph-long descriptions of roadside foliage that are a single sentence. Austen dealt in interpersonal relationships and power imbalances; Baker is keen on totemic objects like James's collection of seashells.
The plot didn't work for me on its own; there were several key moments of this book where something was revealed with great pomp and circumstance that I'd figured out long before, and I honestly couldn't figure out whether Baker meant the scenes as actual plot twists or satisfying resolutions of what the reader had begun to suspect. I found Sarah likable, but there was something so soft-focus and arbitrary about her relationship with James that I didn't care much about them.
Obviously I considered this novel thought-provoking enough to finish, but I didn't find it a success. Read it if you like current woman-oriented literary fiction; skip it if you like Austen's wit.