Today there are so many thrillers since almost everybody seem to be writing thrillers. So you have to be damn good stand out from this crowd. This book is the second installment of the Smiley Series but it has nothing to do with the Cambridge Circus and its agents. It's more of a probe into Smiley's character as he plays a detective as a favour for a friend. You can skip it and you'll lose no substantial information when the series as a whole is considered. But this is a damn good novel and really quick to read. So I didn't see any reason to miss it.
P. S. The thing that I found interesting here has actually nothing to do with the Smiley series but with Game of Thrones. GRR Martin has directly borrowed without modifications a few terms from this book which made crucial GoT stuff, for eg. The Long Nights, & ...they were called crows b/c they dressed in black... and a few other things.