Good literary fiction novel, not very good fanfiction. The writing is clear and evocative, the characters well-drawn. There is a little too much head-hopping: a semi-universal third person point of view works in Austen because she had an ironic detachment from the characters and the ability to raise an eyebrow even at the flaws of her heroines, but Baker goes so deep into her protagonist's thoughts and feelings that it's truly jarring when we're suddenly involved in another character's brain, but the writing itself is very sharp.
(Some spoilers below.)
But the book is fanfiction, regardless of its literary quality - it's set in the universe of Pride and Prejudice, using that book's main and secondary characters, the characters that were only named and not really given characterizations, and original characters that existed in the P&P background but were never actually named or described. And I can't help but evaluate it based in part on how well it keeps the characters in character and how much it takes them out of character, and it really doesn't hold up well here. Mrs. Bennet, Jane, Lydia, and Kitty are kept in character with little trouble; Mr. Bennet shows an entirely different side to Mrs. Hill due to their relationship, but it doesn't seem too out of character; other characters are not recognizable. Elizabeth is particularly hard-hit - for most of the book, she's just a generic rich girl; after she marries, the characterization takes a particularly problematic turn, with her very seriously telling her maid that, "You must understand that I am anxious to be quite as he would wish me," before having Georgiana's lady's maid be the one to dress her hair. It seems very unlikely that the Elizabeth of P&P, who dearly loves to laugh and refused to marry Mr. Darcy until he proved his worth beyond his wealth and property, would worry this much about being fine enough in appearance. She also takes her maid's decision to leave her employment much worse than I'd expect, becoming suspicious and annoyed at the possibility that Sarah could be unhappy at Pemberley and enlisting Mr. Darcy to interrogate her and browbeat her into staying. By fanfiction standards, this is a big problem. (Mr. Collins is also out of character, made to be more considerate and thoughtful in order to make the Bennets look worse for rejecting him, but this is a much less big problem.)
But what about Sarah, the actual main character of the novel? An orphan who came to work for the Bennets as a girl, Sarah has a restricted and unromantic life at Longbourn. My main issue with Sarah is that she's so resentful: sure, it's understandable why working day in and day out for rich people with no concept of who you are or what you might do besides look after them would make someone resentful at times, but it's like 330 pages of resentment. She doesn't take pride in her work. She doesn't have moments of high life below stairs, apart from her briefly-depicted romance with James. She doesn't plan for a career or try to move up in the servant hierarchy. She doesn't make jokes about her employers. She constantly notices how dirty and sloppy everything is, as though she weren't used to it. In this, the book seems to be falling into modern-middle-class-person-in-the-past characterizations, my least favorite aspect of historical fiction. I can get past the modern writing style (even though I wished that this read like Austen filling in the other side of P&P), but not the modern mindset.
The historical accuracy of the book is somewhat hit-or-miss. Even without the acknowledgement at the end of a number of books used for research, it's clear that Baker did a lot of reading: day-to-day life is depicted in painstaking detail. However, some problems stand out to me. The book is set around the publication date of P&P (1813), but Elizabeth is described as wearing "short-stays" [sic], when the long corset had come into regular or at least fashionable use by this point. A flashback to the 1780s or early 1790s has women wearing "corsets", a word not used in English at the time. (There are a number of other anachronistic words - right now I can only recall "rucksack" and "backpack", but examples are sprinkled throughout the novel.) When Sarah goes to Pemberley, she's given an already-made gown of black flannel as a uniform, but a) black uniforms were not a part of maidservants' dress until the end of the century, and b) lady's maids, in any case, were able to dress almost as well as their mistresses. Her work there is entirely making and repairing underclothing (shifts and petticoats), the descriptions of which sound a lot more like turn-of-the-century underwear than that of the Regency. These aren't serious flaws, but they did throw me out of the book every time I got to one.
Less definitely inaccurate, but I have to wonder if it's realistic for a relatively wealthy family like the Bennets - who only seem to be in a precarious position because we look at their lack of saving for the girls' dowries front-and-center due to Elizabeth's viewpoint - with seven family members to be served by only an elderly butler, a housekeeper/cook, two maids, and very occasionally a footman. Labor was cheap.