I should say right now that if you're simply looking for more Mr. Darcy, this is definitely not the book for you. While it's true that much of the action of this book mirrors the drama of Pride and Prejudice, it's strictly the backstage version of that great story. We see almost as little of the Bennet family in this book as we see of the Bennet family's servants in Pride and Prejudice. And when this book is compared to Downton Abbey, it's solely the below-stairs part of that great show that invites the comparison.
The third person narration centers around Sarah, a maid in the Bennet household who is treated decently by both the family and by Mr. & Mr. Hill, the primary servants. The prose makes a good effort at passing for period writing, which means the reader gets the best of both worlds: a more linear and straightforward sentence structure of modern narrative, combined with all of the charming archaic language not commonly found outside of Regency novels. In other words, there is nothing glaringly contemporary about the writing, which is one of the greatest downfalls of historical fiction, in my considerable estimation.
Beyond that, the writing is actually quite good. Not necessarily of the "I want to read these passages over and over for their sheer beauty" persuasion, but of the more subtle "this writing is carrying me along quite beautifully through the story without jarring me out of it with any language missteps" variety. It is consistent, occasionally elegant, and always imbued with the flavour of the period.
There's also an earthiness about the book lacking in Austen, for poor Sarah must deal with those things in life with which the Bennet girls would never sully their minds or hands. One instance rendered quite well in the book, pertains to that famous scene where Elizabeth Bennet walks to Netherfield across muddy fields, dirtying her petticoats and earning the admiration of Mr. Darcy: "If Elizabeth Bennet had the washing of her own petticoats, Sarah often though, she'd most likely be a sight more careful with them." Indeed.
Still, life is generally good for Sarah: she has a position in a modest country home, and though her mind fantasizes about escaping, she is realistic enough to know that striking out on her own for London could be the ruination of her, for how little she knows in the way of the world. And so she toils on, guiding young Polly the scullery maid in the ways of the household, and forming her mind by borrowing books from Mr. Bennet's library, when one day a mysterious young man blows in seemingly on the wind. In a place where most young men without fortune have been conscripted for the Napoleonic wars, James stands out, and Sarah takes it upon herself to discover the enigmatic past of the new footman and learn why the Bennets might hire him without a reference.
To say much more would be spoilerific, but I will say this: Mr. Bennet harbors a dark secret from his past, George Wickham plays not just the rake but a sadistic and pedophilic one, and Sarah has miles to go before she sleeps. If you are any kind of reader of historical fiction, this book should land on top of your To Be Read pile, regardless of your feelings about Pride and Prejudice. I prefer to think of Longbourn as a terrific companion piece to all of Austen's novel, showing the behind-the-scenes drudgery of daily life for all of those people not fortunate enough to have been born above stairs.
NB: I think this book is poised to be a big book for the fall publishing season here in the US. It's already been published to great acclaim in the UK and Knopf will bring out the American edition in October.