Astoundingly good. I've read Le Carre books before and really liked them, but this one blew my socks off.
George Smiley is a former spy master who's been forced out of MI6 (the Circus) after some bloody debacle in the Eastern Bloc. But he's summoned back to secretly investigate suspicions of a mole at the highest level of the Circus.
So the plot's pretty straightforward but this is head and shoulders above the run of the mill spy thriller. Le Carre was a spy himself, and was forced out when another high level mole Kim Philby blew his cover, so the spycraft described is very convincing. What's apparent is that it's not glamorous in the least. A dirty, lonely job where you have to lie and betray people, and you can't even trust your own side. Being a spy doesn't appear to be very good for you psychologically.
Smiley's a fantastic character. Middle aged, tubby, polite and unassuming. He's repeatedly cuckolded by his beloved wife - we never meet her but it sounds like she badly needs to be shown the pimp hand. But you can see his skill and subtlety as he winkles information from various other damaged people who've been sacked from the Circus.
A lot of the book's in flashback from these interviews, and it has bits in Hong Kong, India, Czechozlovakia. But it's the portrayal of Britain in the 70s - London in particular - which is the most striking. Seedy bedsits, shabby people, soggy raincoats and bad food. The sense of place and time jumps off the page.
It's told in quite a subtle and rich way and I sometimes found myself re-reading pages to get everything out. But it's certainly rewarding. By the end when you see how wonderfully all the plot points have been put together, you realise it's pretty much the perfect thriller.
Now, I'd seen the TV version of this years back, so I knew who the mole was. Actually I knew who the actor was, but luckily I couldn't remember who he played. And it's impossible when reading this not to think of Alec Guinness in the Smiley role, even though the book makes it clear he doesn't wear a hat (his wife says it makes him look ridiculous.) There's a film version out soon from the director of Let the Right One In. Gary Oldman as Smiley, which I can definitely see. But I'd urge you to read the book first.