Here's an interesting question:
What makes a "fun" book vs. an "important" book?
That is, what is the difference between a book that is pure entertainment and one that people say you "should" read because it will make you a better person or give you a new perspective on life.
Are they really two discrete entities or can an important book be fun as well? And perhaps more importantly is the opposite true - can a book described as "fluff," a title given to books such as Fifty Shades of Grey - be given the title of "important?"
Let's examine a little bit of what we mean by whether a book is fun. I think a more sour-oriented soul would describe fun as meaningless escapism, something you read that caters almost directly to your preferences, and does not try to challenge you in any meaningful way. I feel that romance is the most frequently and off-handedly referred to genre in this way. Because there was a greater social stigma towards romance as a worthwhile genre. Recent YA romance novels and adult romance novels, like the very aptly named Twilight and additionally aptly named Fifty Shades of Grey (and sequels) have brought romance back into the public eye, and less away from what your mother secretly reads and you just grew up looking at the shelves wondering what are those? (that might've just been me)
These books are "fun" but they are also pretty fun. It's escapism. Sometimes, you don't want to think, but you don't want to just drain your brain and watch a movie or tv. You want to read something stimulating, but not necessarily challenging. There's nothing wrong with this - what do you think a massage is to your body? That's what reading fun for your brain is. It doesn't really make you stronger, or faster, or anything like that, but it relaxes you so you can go back to the gritty real world (or more important, depressing books)
It's not really this way anymore, but there's still some culture of "oh fantasy and science fiction is for losers" out there. In my experience, this is mostly in academia -- there's many a "creative" writing prof out there who derides genre fiction as pointless and worse, bad.
Search twitter and you'll find plenty of people sharing stories of their creative writing profs saying that they will fail someone for writing genre fiction, or that you shouldn't look up to or be impressed by Neil Gaiman because he's a genre writer. And *gasp* he writes comics, too! Truly, the man must be of the lowest calibre of author.
So we can agree that fun books can be important. But can important books be fun? You'd be surprised how many lit professors don't like hearing certain books referred to as "fun."
I'm not going to argue that there's no value in a purely "important" book, one that, like some grand, depressing Russian epic treatise on communism, will open my eyes to the darkness of our own world, and make me a better (or at least more thoughtful) person. There is great value in books like these. But even those dark books have to have some fun in them to be entertaining, otherwise no-one would have read the damn things. War and Peace might be one of the most "important" stories ever written, but if it wasn't fun at all to read, if it was just some high-school textbook that was like "war is bad" and "peace is good but it never turns out the way you think it will" "now feel sad for 800 years." No one would read it, and no one would recommend it, and it wouldn't be considered a great work of art.
After all, nobody refers textbooks to each other to read, even if they are "important" and textbooks clearly are. They are all "important" because they are all trying to teach you something, just like War and Peace. They just don't have a single ounce of "fun" in them (unless you're one of those people who likes math).
And, at the risk of upsetting everyone who read this far, it's all moot anyways. What your book means to you, the lessons you think you're trying to teach people, or the lessons you're NOT trying to teach people, all don't matter because no one that buys the book will be you. What I mean by that is that everyone has a different perspective on life, and whatever interpretation of your book that they have is valid.
If you write a book that is nothing but erotic sex in zero g (shh I totally don't have a collection like that) and someone decides after reading it that it's a treatise on the mistreatment of ants by scientists, well, that's a valid point of view. If that's what you got out of it, then great. But not everyone's going to get the same things out of the same book. I'm sure that somewhere out there is a person who read "War and Peace" and decided afterwards that War was pretty radical. There's probably someone who thinks that it's secretly foretelling 9/11.
So if someone says your book is just "fun" but they don't think it's going to change anybody's lives, they're wrong. It might not change their life, but they are only one of over seven billion people, probably at least one of which will find your book, however vapid, life-changing. There are books that I read when I was a child that changed the way I view the world and I would not read them again if you asked me to, because now they seem terribly written and just full of crap.
By the same token, when I read Great Gatsby in High School, as many Americans did, I didn't think it was fun OR important. I thought Gatsby was a huge loser and I don't remember learning anything from it. Rereading it as an adult gave me a lot more.
So if you ask yourself, "Am I writing a fun book, or an important one"...
Just don't ask that question. Because you don't decide what your book is or means. The people that read it do. (And not just because they are giving you the money so you just have to accept what they say.) If 90% of adults see your book as crap, but one teenager says it changed their lives? That is an important book.
So go write the next War and Peace. But please, make it a little more entertaining. For my sake. :D