I have a new rule: books I read for explicitly edifying purposes should be textbooks. I am allowed to read other books, but they go against entertainment in the ledger*, and don't count as learning any more than Terry Pratchett, Dilbert or Project Runway. **
I'm doing this so I can't convince myself that I understand a field of study when I don't. This can happen for a variety of reasons, but I think an especially common vector is being a part of the skeptic / pro-science / atheist crowd on the internet. As part of this crowd, you're introduced to a very shallow reading of a variety of enormous subjects (evolutionary biology, statistics, cosmology, logic, philosophy...), and especially if you take up skepticism / pro-science / atheism as political positions to be argued at all costs, you're going to abuse what little knowledge you have in defence of your protected beliefs and say a lot of stupid things. I shamefacedly hold my hand up and admit to doing this.
(A note to people who persist in doing this: it's piss-annoying, and if you ever grow out of it you will regret it more than any youthful fashion-blunder you can imagine. If you stop now, you will regret it a little less.)
There are slightly less embarrassing ways to scratch the surface of a subject and think you've exhausted it. A lot of popular science literature seems to do a good job of making the reader feel they've learned something, and a bad job of imbuing the reader with a sense there's more to learn. When you finish a pop-sci book on a subject you're unfamiliar with, you feel like your mind has been expanded, and compared to your still-unfamiliar peers, you can feel like a resident expert. It's as if all the condensed insight from the field has been given to you, and the messy underpinnings are something you're fortunate enough to not have to worry about.
It's easy to have an armchair understanding of a subject. You might know considerably more about that subject than the average person on the street, but that is a ridiculously low bar. There's a sense, I think, that moving from armchair understanding of a subject to pursuing it as part of a course of formal study is simply "filling in the blanks". My experience of diverse formal study is that while this is sometimes the case, on many occasions, those "blanks" turn out to collapse into vast crevices, and the fall can be very uncomfortable if you're not prepared.
As a result, I've decided the armchair is an unsafe place to theorise from. If a subject is important enough for me to be interested in, it's important enough for me to work through an undergrad textbook on that subject. This won't make me an expert in all these subjects, but that's not the point of the exercise; the point is to learn how much ignorance I have.
The first area where I've decided I don't know enough is philosophy. Courtesy of Abe Books, I've obtained Ernest Lepore's Meaning and Argument, Norman Melchert's The Great Conversation, and for good measure, Bertrand Russel's History of Western Philosophy. This seems enough to be going on with. As I finish each, I plan to report on what I feel I've learned from them.
* I don't actually have a ledger where I document all my media intake and meticulously label how worthy it is, though now the idea's been floated, I'm seriously considering it.
** I don't actually watch Project Runway. There is not enough helium in the world to float this idea high enough to make me consider it.