The Honourable Schoolboy is John le Carré's eighth novel. Published in 1977, it follows Tinker Tailor Soldier Spy and is a sequel to it.
It is about George Smiley's ongoing quest to track down the Soviet spymaster Karla, this time via connections in Hong Kong, though Smiley is not the central character in the main plot. Gerald (Jerry) Westerby, a journalist and sometimes-spy first seen in Tinker Tailor Soldier Spy, drives the action in Hong Kong. Smiley sent him there to look into a money laundering operation in Laos that had previously been investigated by Circus operative Sam Collins, though the investigation was suppressed by the character revealed to be the mole in Tinker Tailor Soldier Spy.
Westerby is dispatched to southeast Asia, where he tries to ascertain Drake Ko's connection to Karla.
The Honourable Schoolboy is the second installment in le Carré's "Karla trilogy," between Tinker Tailor Soldier Spy and Smiley's People. Le Carré wrote that he originally intended the Karla saga to take place over a long series novels but later changed his mind and only wrote three. This explains why The Honourable Schoolboy does less to expand on Karla's backstory or to advance his plot than its predecessor and successor.
The most interesting parts of The Honourable Schoolboy are those that deal with how the Circus handled the revelation that one of its employees was a Soviet mole. (That's not really a spoiler for Tinker Tailor Soldier Spy; the fact that Smiley is attempting to sniff out a mole is explained on that book's dust jacket.) Westerby's actions in Hong Kong are interesting as a standalone story, but don't quite rank among le Carré's best.
If you're looking to work your way through le Carré's entire oeuvre, this is a good read. If you're interested in a more direct sequel to Tinker Tailor Soldier Spy, you can skip straight to Smiley's People.
The Honourable Schoolboy was not adapted as part of the BBC miniseries featuring Alec Guinness as Smiley, apparently because the costs of shooting in southeast Asia would have been too high. It is not likely to be adapted into a sequel to the 2011 film version of Tinker Tailor Soldier Spy, primarily because the characters of Westerby and Collins are so integral to the story's plot but were merged into a single character in that film. As Auduster notes, it was adapted for radio by the BBC; Simon Russell Beale starred as Smiley.