The premise is Locke Lamora is an orphan born in a kind of fantastic version of Renaissance, Italy. Specifically, Venice. It is a spectacularly crappy city where children are hung to die for thievery, the rich live in extravagant decadence, and the poor in absolute squalor. A "Secret Peace" exists between the thieves of the city and the upper class where the former can prey on the middle as well as poor without reprisal as long as they leave the elite alone. While this keeps them away from the truly wealthy, it makes up for it in quantity.
Locke is no ordinary thief, though, but a member of the Gentleman Bastards. It is a sect, in the most literal sense, of thieves who worship the God of Theft. Raised by Father Chains, they exist for the purposes of pulling off the most dangerous of heists as well as bizarre of scams. There's no real point to their activity but to honor their god and they only live comfortably by masquerading as less successful thieves. Even so, they've impressed the local mafioso head who wants Locke to marry his daughter. It's all set up as a funny humor-filled gentleman bandit caper book before things go HORRIBLY wrong.
It's difficult to explain just how engrossing the writing of Scott Lynch is. He crafts a beautiful yet repulsive world that is at once monstrous yet completely believable. The essence of grimdark, to me, is that it casts a light on the diseased portions of human history as well as the moral repugnance that often underlines the "Good Old Days." Here, this is the filthiest version of the Renaissance you'll find outside of the Black Death. There's sympathetic characters left and right but they are all capable of terrible things at the right prodding or carry the views of their station. The monsters are also humanized.
One character I love in this book is Capa Barsavi, who is a monster capable of murder at the drop of a hat and wiped out an entire family of his rivals in a horrifying manner decades ago. Yet, he is a devoted father and a friendly patron to our heroes (even if he would kill them if he knew how much they were skimming). Opposing him is the mysterious Grey King who wants revenge for perfectly understandable reasons but is going to tear the entire city down around the citizenry's heads.
Locke is just the right side of brilliant as well as insufferable. Possibly the greatest thief of his time, he's arrogant enough to be genuinely flawed as well as vulnerable. Numerous times during the book, Locke gets himself in trouble because he just can't help showing off. He still has a vulnerable side as he loves his family and brothers to a fault. You feel his grief and pain when he loses those he cares about. His companion Jean isn't quite as well characterized but is the reason why our protagonist hasn't been killed due to the much more pragmatic but less imaginative head on his shoulders. The Watson to his criminal Sherlock Holmes meets Arsene Lupin if you will.
The depiction of magic in this series is also great as it is, in simple terms, undefeatable. The mages of this world are capable of doing whatever they want to whoever they want because they can destroy anyone who harms one of their kind. They are a monopoly on the equivalent of atomic weapons so they just wander unimpeded by everyone around them. The only thing keeping them from ruling the world is they don't want the responsibility versus extorting outrageous fees from their employers.
Interesting, I have to give this book credit in a rare area. Grimdark is usually a genre associated with hyper-masculinity and misogyny as an accepted (albeit historically accurate as part of the horror) quality. Scott Lynch's world is far more progressive with women at every level of society and equally as dangerous as the male. There's some mild sexism but this is a rare book which passes the Bechdel test despite being from the perspective of a male master thief. It also builds up a character I absolutely love in Sabetha, who sadly doesn't appear on page until The Republic of Thieves.
The plot is, deliberately, as convoluted as an Ocean's 11 movie with the big difference about half of the team is not going to make it through. Scott Lynch is not afraid to murder large portions of the cast and that includes people you wouldn't think would be harmed. The mood whiplash from comedy to tragedy is immense and why this book is so damned memorable. It is also entirely self-contained, as mentioned above, and worth it as a one-and-done. The story was continued afterward but there's no hanging threads.