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A perfect spy is one from the end of the Cold War, and it is halfway between a thriller and a serious novel, so it might be another good entry point.
The really early ones you can skip until you're a hardcore fan: A murder of quality, Call for the dead, The looking-glass war, A small town in Germany.
The more recent books are a mixed bag. As reading_fox says, The constant gardener is very good, but others like The Mission song and The tailor of Panama are flimsy potboilers; others again are just leftover bits and pieces of Cold War warmed up for re-use.
The first book Smiley appears in is A murder of quality, but there's no need to read that first: it's unlike all the others anyway, as he is doing a sort of Hercule Poirot impersonation solving a murder in a public school. Some of the characters are former spies, but it's not a spy story. He appears in the margins of several other books as well, but the ones in which he plays a central role are the so-called Karla trilogy. It's not essential to read these in order, as there's a lot of recapping, but if you're going to then you might as well start with Tinker, tailor, as it's easy to find anyway.
I thought that Call For The Dead was the first Smiley book. It is still not quite a spy story although we do get some bits of Smiley's past.
Starting with The Spy Who Came In From The Cold was Le Carré's breakthrough book. It also slightly retconned Smiley's history as it had been revealed in the previous two novels (Call For The Dead and A Murder Of Quality). Smiley is a minor character in The Looking-Glass War so I wouldn't recommend that as a starting point.
I would start with The Spy Who Came In From The Cold and then move on to The Karla Trilogy.
A Perfect Spy is perhaps Le Carre's best work and I think best appreciated after reading the novels that came before it and getting a flavor of le Carre's viewpoint of how the whole business of spycraft is absurd in many ways and really just a game between the countries involved.
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