Rosetta is, as the front cover suggests, a Hoshi story, revolving heavily around her passion and profession of translating alien languages, and also delves a little deeper into the character regarding her experiences after being abducted by the Xindi.
Reading the Enterprise novels chronologically, I'd have to say this is the best book of the series at this point. Set towards the end of season 4, it has a fair amount of history and running plotlines to use to give the die-hard fans some fanboy moments, however in my opinion, didn't really capitalise on this. There are some references to major points in some of the characters' history (Travis's upbringing on Horizon, the aforementioned kidnapping of Hoshi), but doesn't do much scratching beneath the surface. There are a number of references to a Lieutenant Hess in engineering and faffing about while she's ill or causing problems for Trip, where at this point in the series some unresolved tension between replacement chief engineer Kelby and Trip may have been more interesting. Also, there a few factual errors and inconsistencies as well as a number of typos and grammatical errors which do detract from the book a little. This book is another instance of Hoshi losing her surname at various points, simply being referred to as Ensign Hoshi, and Crewman Cutler (not seen since in the series since the untimely death of the actress) suddenly reappears as a nurse in this outing.
It's for this reason I only gave the book three stars, as it feels like Dave Stern may not have actually watched the show with any great passion or attention to detail. Characterisation in particular seems off sometimes with T'Pol not really being the more emotional, flawed character we come to know after season 3 (and certainly not after her marriage and mother's death, which also lead to Trip's transfer to Columbia - also barely mentioned). With season 4 being as rich in storytelling as it was, with all its nods to the earlier series, the lack of references to the happenings of that season (or indeed that the MACOs only get a one line reference in the entire book) feel like the story was shoehorned into this particular point in the timeline for no reason and stands apart from the season because of it.
These niggles are outweighed by the fact the book is easy to read - the secondary characters are memorable enough to not start getting them confused with one another, there isn't a heavy emphasis on technobabble (which is always good!), and we do get to delve into the lesser used characters of the series a little more. Travis has a fairly decent subplot, while not doing much to develop his character, it's always a pleasure for me to see the character get some time in the limelight. Reed shows a bit of a cheeky side I think with regards to the way he handles his investigation and wasn't so stiff upper lip about everything, without misrepresenting the character of the show. And of course, we get insight into Hoshi who is still deeply scarred from her experiences with the Xindi, which gave the character some deserved depth.
The story itself contains a number of threads running concurrently which all come together rather well, and the main plot itself involving the Antianna keeps enough mystery and intrigue to keep your interest to the end. The conclusion feels a little rushed, but isn't necessarily a bad thing as it happily leads into the "Demons/Terra Prime" two-parter.
Worth the read for any Enterprise fan who wants a bit more than just the four seasons we got.