Ivan Vorpatril is one of Barrayar's most eligible bachelors and notorious rakes, but now in his mid-thirties he is finding his life of chasing women and partying is no longer as satisfying as it once was. On assignment to Komarr, his path crosses of that of two fugitives from a coup on Jackson's Whole and his attempts to help only make things worse...and change his life forever.
The most interesting thing about the Vorkosigan Saga has been Lois McMaster Bujold's willingness to experiment, switch protagonists and POVs and generally not sit still and bash out a load of action-adventure novels. Her willingness to put the series on hold for years at a time until she has a good idea for a new book has also helped it retain a high level of quality.
Captain Vorpatril's Alliance is one of the lighter novels in the series. It is a romantic farce with an underlying adventure story and also dwells on the notion of ageing, growing up and maturing, a theme of Bujold's that she returns to repeatedly in the later books in the series. Using Ivan, Miles's womanising cousin with no interest in settling down, to explore this theme is extremely effective. It would have been easy to have done a "growing and learning" story in which Ivan suddenly mans up and accepts responsibility, but this would not have been true to the character. Instead Bujold develops Ivan's character (and, we realise, how she's been developing it subtly in the background all along) naturally and much more convincingly, by having him fall for a woman who seems to be right up his street (superficial and pretty) but whose hidden depths and complex background make her a lot more interesting.
These elements of growth and change are accompanied by some quite uproariously hilarious scenes, some nice catching-up moments with old characters who we haven't seen for a while (most notably Simon Illyan) and some more musings on the changing nature of Barryaran society, which are all handled quite well.
On the downside, the novel is a bit too long (over 500 pages) to support a slight premise and the lack of some well-motivated villains (we never even meet the bad guys who set the whole story in motion) and there are a few too many scenes of Tej's family scheming or Ivan feeling overwhelmed. A bit more of a serious editing pass to streamline the book would not have gone amiss.
Captain Vorpatril's Alliance (****) is not one of the best books in the series and could be a bit better paced, but it remains well-written with a refreshing focus on the characters and how they have evolved over the years, with some nice SF flourishes and very funny moments. It is available now in the UK and USA.