Candidus said:
If you really want to understand, take a risk. Spend some money on the Eisenhorn trilogy from the Black Library (unless you're one of those really cool people with exceptionally refined taste who look down on the Black Library... /sarcasm). There is no finer science fiction out there, in my opinion. No superiors at all, only peers.
W40K is written on a lot of levels. It's a multifaceted universe with a huge ensemble of contributors that are relevant to a wide range of audiences. Some are repellant to me. Many are not.
Honestly, I wouldn't go that far. The Eisnehorn trilogy is good, but it's got some flaws.
First off is Eisenhorn himself, in two prongs:
The books are written in first person, and to be honest, it's not Abnett's strong suit. He literally says as much in the introduction. We loose a lot of material over the course of the books because everything is from Eisenhorn's perspective. While it works as a cop out, it also means we're left to infer any character flaws he may actually have because he sure as hell isn't going to put them in his report.
The lack of personal flaws is a pretty serious, and pretty solid problem. It's only in context with the glut of really genuinely badly written protagonists that saturate most media these days that this one gets lost. He is constantly surrounded by more flawed, and to an extent, more interesting inquisitors, while he marches on, the designated hero, accruing scars to demonstrate his prowess. This only gets worse as those scars, injuries, and lost equipment don't really work against him in the long run.
I'd get into serious spoiler territory here, but off hand, thinking of any serious failure and setback he suffers, and you can point to an event usually in the same book, where that new disadvantage actively benefits him.
At the same time we've got Bequin, who goes from being an actual character in the first book, to a random background object that Eisenhorn would like to bang, but can't so he angsts about it... the aversion people feel towards blanks mysteriously vanishes between the events of the first and second book, along with her presence... she does the mother hen thing briefly, and then... stick a fork in her, she's done. She wanders around aimlessly for the rest of the book and well into the second one.
Now... it's possible he's simply forced himself to overcome his natural aversion to blanks... but he never says that (to my recollection). It would make sense, given that the Distaff is basically a tool for his personal usage. But, it's not in the book, so this is more wild speculation than legitimate analysis.
With the caveat that I haven't finished it, and I've already run into some stupidity in the writing, I'd actually hold up at least the first Ravnor book as superior. Ravnor himself is a much more flawed character than Eisenhorn, and the shift between first and third person actually gives the first person elements some actual context.
All of this said, Eisenhorn is a really good trilogy, and some of the best tie-in fiction I've seen, but it really doesn't rise to the level of best of sci-fi among actual science fiction classics.