You cannot be a good writer unless you read. This is pretty much a given, yes? You need to know how other people write so you can 1) learn from the pros and 2) figure out your genre/market. It's very much an osmotic kind of process—you'll absorb a lot of information about craft and style without really realizing it. Or at least I did.
But I'm of the opinion that you can't just read the good stuff. Sorry, but you can't. You have to read crap as well, and by "crap" I don't mean "trashy books". There are a lot of well-written trashy books. You need craft to write fluff novels, weekend reads, romances, and off-the-rack thrillers, just as much as you need it for literary fiction, commercial fiction, and the like. It's a different style, yes, but it's still craft.
No, what I mean by "crap" is anything containing purple prose, talking heads, repetitive sentences, clichés, and anything else that makes brains leak out of ears, eyes start skimming, and publishing professionals post "don't do this" blogs. It's the kind of stuff amateurs write, the kind of stuff you find pages of on Fanfiction.net, the kind of stuff we all write at some point. And reading it can be very educational.
If you* read a badly written book while unaware of what constitutes Bad Writing, you may or may not notice. At the very least, you might not realize why you've started skimming, why you don't want to pick the book back up again, why you're confused, and why you're left unsatisfied at the end.
If you read several badly written books while unaware of what constitutes Bad Writing, you'll start to see the trends. You'll notice the technical mistakes (clichés, dialogue tags, etc.) and you'll notice the plotting weaknesses (climax comes on too suddenly, Chekov's Gun is missing in the first act, etc.), and, I hope at least, you'll notice where you're doing similar stuff in your own work. You'll probably even have some ideas about how to fix the problems, because you'll start thinking things like, "Why didn't Bob hint about Immensely Important Plot Point instead of just springing it on us?", "Where the heck did Josephina come from?", and "This would read so much better if there weren't all these adverbs."
If you read a badly written book while aware of Bad Writing, you'll be more attuned to the errors, you'll likely notice more of them quicker, and you'll be thinking of solutions (or possibly saying, "I could do this better!" That's also good.). Reading several bad books while aware of Bad Writing is not particularly advisable for your sanity. You'll know when you should stop—and I by all means advise mixing Good and Bad, emphasizing the Good by a long shot.
Something else I've found helpful in terms of learning from Bad Writing? Editing it. It's one thing to read a book and think of fixes for a problem, and another thing to read a book and have to fix the problem. Or at least suggest fixes for the writer. I find I learn more … concretely when I have to write my ideas up, knowing that my ideas might be accepted and used, so they'd better be good. Also, I may not always be aware of, say, my adverbs, but I'm definitely aware of somebody else's, and editing them makes editing me easier.
Of course, "crap" is a subjective term. You may find even the best written "trashy novel" to be absolutely horrible, and that's fine. You're probably not intending to write that kind of book. You're destined for other things. Or you might spend years gladly, willingly, enjoyingly reading what I call "crap" and not care, not notice, not want to change your habits. That's also fine. We'll agree to disagree. But (and this is my main point, take notice): if you want to improve your own writing, reading flawed writing of any sort** will help.
* And by "you" I probably mean "I". I'm extrapolating, generalizing… I know this, and if my ideas don't fit your reality, my apologies.
** I know, I didn't mention short stories, screenplays, stage plays, blog posts, or fanfiction, but they also count.