Second, it has an unusual sense of humor for a mystery novel (apparently it is technically "crime fiction"). I've already posted about all the birds in the book (the title, which refers both to a poem in the first pages of the book and to the name by with one character referred to the dead person and which the dead person, prior to her death, sometimes used to sign emails; the protagonists -- Cormoran (sic) and Robin; Cormoran's dead mother, Leda; Robin's fiance, Matthew (10:29)). In addition to the bird references in names, Cormoran goes to a bar called "Feathers" at one point, and then of course the whole damn thing involves the fashion industry because Cuckoo/Lula was a model.
HEY, SPOILERS! Look out, there's a serial killer with a possible personality disorder who might push you off something high!
Cormoran's got a dysfunctional family background. Mom was a "supergroupie" and died from heroin overdose. Dad is an aging rock star who was never a part of Cormoran's life, but since everyone can look in wikipedia, everyone knows that Cormoran's dad is that aging rock star and they bring it up all the time, much to Cormoran's annoyance. He has a sister, married, kids. His on-and-off again girlfriend since his initial years in college, Charlotte, exits the book on the first page or two and we don't see her again (a couple texts and a phone call later and that's it). Charlotte comes from money, and over their decade and a half being on-and-off, he learned a lot about rich people.
The dead girl was adopted by a well-off family (dad made a bundle in electronics, died, and the trust fund is mostly in that company and it got hit hard by the recession), as were two other boys, one of whom died in childhood (friend of Cormoran's). The other boy, grown up, hires Cormoran to find out what really happened to his sister. The sister did _really well_ modelling -- had 10 million at time of death, which was probably more than everyone else in the family by that point, put together.
In addition to the ins and outs of the fashion industry, Galbraith uses these characters to ably depict London as a place where families which are sliding _down_ the wealth ladder interact with up-and-comers: people who married money (the Chillingham sisters, Tansy and Ursula), people who are desperately trying to destroy themselves (Evan Duffield), people who are succeeding at Second Fortune (Freddie Bestigui, who inherited money and then made more in movies; the dead girl, who was adopted into money, and then made more modeling). In the background are professionals who provide services to the money (the lawyers, such as Ursula's husband Cyprian -- man, the Georgette Heyer slang jokes here are amazing -- and John Bristow and Tony Landry, who are also busy sliding down the wealth ladder; but also Matthew, who IIRC is in accounting).
Cormoran doesn't "fit" into any of these categories. While his dad is rich, he never saw any of it. His ex is rich, but that is over. He's not _getting_ rich doing what he is doing. When anyone in the World of Wealth deigns to notice him (mostly the night that he winds up sleeping with Ciara, who, like Cormoran, has protective coloration to prevent people from noticing how intelligent she is), they mostly slot him into the minder/bodyguard category.
Robin fits in, technically, even less well (from Yorkshire, a secretary/administrative assistant), but she turns out to be such a chameleon she fits in well everywhere.
Black cabs and car service (and the drivers of same) function as another important group of characters.
So, themes and characters are strong. How's the plotting? Intricate. I didn't spot any errors, and given that this is one of those stories where the detective goes around and collects every bit of evidence and everyone's schedule on day of to figure out what doesn't fit and why and then breaks a bunch of alibis and pieces together what really happens, that's good. The depiction of Cormoran's interview techniques were really, really compelling and excellent.
I'm interested in reading _Silkworm_, and pleased to learn that a third book in the series is due out later this year. I highly recommend this book.
(And yes, I am aware the author's name is a psuedonym and who the author really is. If you think about it, that probably explains a lot about why the author chose to depict this transitional wealth category and the people who populate it.)