Diplomatic Immunity is the most recent (2002) volume of the ongoing Miles Vorkosigan series by Lois McMaster Bujold. The series now covers somewhere between 10 and 14 books, depending on how you're counting, and has won multiple Hugo and Nebula awards.

In Diplomatic Immunity, Miles is taking a honeymoon with his wife Ekaterin, whom he met in Komarr and wooed in A Civil Campaign, when he recieves an urgent message from Gregor, the Barrayaran Emperor, sending him to deal with a problem on Graf station. A Komarran trade fleet with a Barrayaran military escort has had conflicts with station authorities and is now being held while several Barrayaran military personnel are under arrest. Miles is soon working with working with his hermaphroditic old friend Bel Thorne (Bel Thorne hasn't appeared in the series since it was fired by Miles as a mercenary captain in Mirror Dance, and is now working at Graf Station.) to investigate the incident that began the whole problem, the mysterious disappearance of a security officer from a Komarran ship. As often happens when there is trouble in the Vorkosigan adventures, a Cetagandan angle soon shows up, but just how Cetaganda is related to the mystery is one of the problems miles must solve.

As even this cursory description shows, Bujold has by this time worked a lot of background into her series and draws on it freely for new stories. Graf Station is named for one of the main charcters in the earlier novel Falling Free, and is controlled by quaddies, the subspecies from that novel who are genetically engineered to have four arms and no legs, and can only prosper in a zero gravity environment. Several allusions to the events of that novel are made. This story also has links, as it develops, to Cetaganda, Miles's previous encounter with the strange genetic manipulation program of the rival Cetagandan Empire, and perhaps the weakest entry in the series. And references to other prior Bujold novels are slipped in. You could probably follow Diplomatic Immunity with no prior knowledge of the Vorkosigan universe, but you would lose a lot of the details, and a lot of the pleasure. One of the strongest parts of Bujold's work is her detailed and varied worldbuilding, and a full appreciation of the future societies she has created requires extensive reading of her novels. The best place to begin reading the series is still at the beginning, with one of the early novels Shards of Honor (first in the series), Barrayar, or The Warrior's Apprentice, the first novel featuring Miles.