According to me, what you're really interested in finding out is:
How do I progress from having an opinion to having an informed opinion?
I'll assume that you're broadly talking about literary and genre fiction and not, say, cookbooks and car manuals. With that, here're my thoughts about the method and the motivation:
1) Antiquity Blues: Go back. Way way back. Read the epics of your culture and of every other significant ancient culture. Brush up on the Greek, Norse, Egyptian, Sumerian, Arthurian, Meso-American, Vedic, and Oriental mythologies and legendariums. These are the earliest stories and collected thoughts of the human race. Everything that follows is indebted to them and identifying their influence on what you read is a pretty good start to appreciating the quality of a book. Read them on Wikipedia if you must, must be aware of them.
2) A Little Knowledge... : can be a pretty cool thing to have. After all, you're practically asking how to be dilettante. Religion and philosophy have long histories, and while it may be impractical to delve deep into the nuances of Kant or Kaballah, it is important to have a brief understanding of the Eight-fold Path or what Nietzsche's Ubermensch is all about. Pick up an encyclopedia or read Sophie's World (if you want your Big Ideas wrapped inside a fictional framework).
3) Eat Your Broccoli: Read all of Shakespeare. Try out at least one book by every "classic" author -- the Dickens, the Austens, the Mark Twains, the whathaveyous. If you're superficial (like me) and don't want to endure the outdated aesthetic, watch a cinematic adaptation. The broader point I'm trying to make with this and the previous two paragraphs is that it is essential to build perspective -- to know enough about what was already present in our world culture and how the text in front of you stacks before it. It adds a second layer of understanding when you spot the archetypes behind characters. Furthermore, it keeps you from getting over-awed by amateurish simulacra that get pumped out in our mass media. There's a reason I'm comparing all this to broccoli. The true turd of literary criticism cannot be passed unless one ingests copious amount of dense, fibrous musings about the human condition.
Interlude: All of this, by the way, is equally useful, if not in substance, then in method, for the purpose of appreciating specific genre fiction like fantasy, science-fiction, or crime thrillers. Are spy novels your thang? Bone up on Graham Greene, Ian Fleming, W. Somerset Maugham and John Le Carre. Does science fiction tickle your warp drive? Asimov, Dick, Pohl et al, are your homies. The point is again -- spit out the roofie-laden punch served by mass-market shills like Coelho and have a margarita with the masters instead.
4) Cool Story, Bro: An overly reductive way to think about your objective is that your purpose is to be unimpressed by as much as possible. Once you start understanding how good the writing (in any genre) can get, most of what is out there will seem mundane. To assuage any digging-myself-into-a-hole type fears, rest assured that there are hundreds of master prose stylists (contemporary and classic) waiting to be discovered and you'll never run short. Simultaneously, you will be able to finally have the vocabulary for expressing why a book is mediocre or bad (or possibly, how it transcends all previous attempts with similar themes). I refer to vocabulary in not the traditional sense, but as a sum total of the texts you've read, where the same themes have been tackled with greater or lesser skill and nuance. If you read long enough, you'll finally get to that hipster wet dream -- giving a one star rating to a universally loved classic with 600 words to back it up!
5) Only The Sith Deal In Absolutes: I end with a word of warning. Let your passion drive you to the extent that it must. However, to achieve your objective is to commit your time and effort to a new way of life. Your free time will be implicitly prioritized for a solitary activity, and much like the TV in Joey and Chandler's living room, everything else will be focused around it. Fun though it may be, you simply can't make a task out of it. Not every bibliophile can or should want to regress into their houses for hours on end, ticking off mental lists of works they need to get through. Make sure to re-read old favorites. Buy mediocre fluff occasionally to keep your reading well calibrated, as well as to have some silly fun. Stop being a cultural leech and actually start writing after a point. Even if it's bad, the fact that you know it's bad is a great sign. A dissonant, interior angst at having read dozens of great books but having nothing to say about them excepting a warm feeling inside is perfectly par for the course. It is far nobler to have too meager and too humble an opinion to offer about the books you read, than being the kind of deluded blowhard who shouts from rooftops and Twitter accounts that Khaled Hosseini is the best living writer in the world.
Homework Assignment: Pick up the complete Asterix collection. Read them with Google and a fine tooth comb. Beyond the silly puns and slapstick, there is a whole other layer of tributes and references to literature, art and music. Figure that stuff out!
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