Silverview

 "And they all lived happily ever after" ends no John LeCarré book ever!  His more usual endings indicate that the challenge described was resolved, for better or worse, until the next problem should arise, likely immediately rather than later. Silverview, the last book LeCarré completed before his death in December 2020, is a well-written, instantly absorbing tale, in this case dealing with changing allegiances.  It's relatively short, with less impact than his greatest works (The Spy Who Came in from the Cold; Tinker Tailor; The Perfect Spy) but I enjoyed it a great deal and recommend it highly.  

Interestingly, there is an Afterword written by LeCarré's youngest son, Nick Cornwall, also a writer, under the name Nick Harkaway.  Years before, he had promised his father that he would complete any work unfinished upon his father's death.  And then Silverview was there, wrapped up in a bundle in a desk drawer.  It had been finished and polished some time previously, yet never submitted to the publisher.  "Why?" his son wondered after reading it, thinking it  "fearsomely good."  They had never discussed it, but Cornwall the younger has a theory, "baseless, instinctive, and not susceptible of proof".  His father was fiercely loyal to the intelligence service in which he had worked, never betraying any of its secrets.  But unlike his earlier novels,  Silverview "shows a service fragmented: filled with its own political factions, not always kind to those it should cherish, not always very effective or alert, and ultimately not sure, any more, that it can justify itself."  And "it is the humanity of the service that isn't up to the task - and that begins to ask whether the task is worth the cost....I think he couldn't quite bring himself to say that out loud." Interesting!