I can only speculate, but Scott Lynch's query letter for this novel may have gone something like this: 
 Dear Agent, 
 Locke Lamora is a thief working with his small, tight-knit gang, the Gentleman Bastards, on another complicated scam to fleece a noble out of as much of his fortune as possible. The Gentleman Bastards have succeeded before, anonymously, a necessity because all nobles are beneficiaries of a secret agreement between Capa Barsavi, leader and master of all the city's gangs, and Duke Nicovante's spy master, the Spider. Merchants are not protected by the agreement, nobles are. The Gentleman Bastard's exploits break this "Secret Peace". Criminals and the very rich and powerful would want them dead if their identities were known. 
 Breaking the "Secret Peace" turns out to be the least of Locke Lamora's worries when Capa Barsavi's power base is threatened by a powerful rival, the Gray King, who is not only wise to the Gentleman Bastard's crimes, but uses that knowledge to press Locke unwillingly into his schemes. Worse, the Gray King has no use for Lamora and the Gentleman Bastards once they've played their part, and he shows it unmercifully. 
 Locke sees no alternative to being the Gray King's patsy for a few rolls of the dice, but playing along costs the Gentleman Bastards dearly. Afterwards, in hiding, Locke wants revenge. When his chance comes, his usual modus operandi, deception, becomes a liability. If others won't believe him, hundreds may die. 
 Thank you for your time and consideration. 
 Sincerely, 
 Scott Lynch 
 Now, my diet of fantasy has been slim for decades. The Potter phenomenon and time has altered the landscape since my glory days, and to be honest, I left fantasy for other genres, except occasionally lately, without ever delving deep and wide. The Lies of Locke Lamora seems to be an example of a swashbuckling, testosterone-fueled branch of fantasy that, perhaps, has its roots in Michael Moorcock's work. (Ye learned scholars of fantasy who beg to differ may scribble and scrawl your outraged wisdoms below.) 
 My query letter probably pales in comparison to Lynch's real one. I think mine does capture the story's gist, but it hardly sounds like fantasy, don't you think? There is world-building, which I had to leave out, but doesn't the story's main elements sound more like a crime novel than fantasy? I'm not a hardened Tolkien lemming who longs for the return of the king and all that noblesse oblige. I'm all for fantasy variations like Neil Gaiman's and China Mieville's, for example, and after this, I'm eventually going to try The Name of the Wind and A Game of Thrones, but I have to admit to being old-fashioned and preferring fantasy that explores the interplay between good and evil, even with doubts, rather than fantasy that celebrates, ah, knavery. It's possible Scott Lynch will have fans who feel he never surpasses this, his first novel. I enjoyed it, I'm not a total sourpuss, but I'm betting that someday he'll write something even better, something that entertains and speaks truths, more truths? 
 PS: 
 I suppose all the detailed description of clothing that individuals wore is part of the world-building, but I'm not sure if it was completely necessary. A bit tiresome at times, sorry. There were a lot of flashbacks that showed the gestation of the Gentlemen Bastards and most of it was entertaining and probably necessary, but once that fateful day in the present action occurred some of that alternating flashback began to gnaw at me, I must admit. 
 PPS: 
 The Gentlemen Bastards are damn good cooks. There are a few detailed descriptions of their meals. An enterprising soul might hitch their wagon to Scott Lynch's by creating a Gentlemen Bastards Cookbook. My cooking skills are not up to the task. 
 Also, like maps? Maybe you could steal some of Lynch's thunder by creating a more detailed map of Camorr. I kept turning to the book's map when the world building description confused me, but it did not always suffice. 
 PPPS: 
 Ladies, if you require strong female points of view, at least one, look elsewhere. If roller coaster twists and turns make up for that, read it. I spent a lot of time wondering how the Gentleman Bastards were going to deal with what came next and next and next...