Viable strategies for selling books in the modern world.
1. Write a great piece of literary fiction that speaks to the heart of the human condition.
Do not do this. They don't sell. They're harder to write, the critics are harsher, and there's little scope for building a brand. Decades after its release, the total number of people who have ever clamored for a sequel to
Gravity's Rainbow: two. The people who clamored for a sequel to
Twilight? Every female on the planet.
2. Westerns.
There is a cult following, but it's entirely made up of 80 year old men in Wyoming and Montana who think the Internet is the Devil's work. Not self-publishing friendly.
3. Sci-Fi.
You have to know science well enough to satisfy the nerd contingent. Not worth the trouble.
4. Mystery.
Too many demands and expectations to master the form overnight. Practically demands re-writing, re-re-writing, and re-re-re-writing as you stick in clues and red herrings and so forth. A demanding lot, and difficult to please unless you've spent a lifetime reading the things, which I haven't.
5. Romance & Erotica
HUGE market, and insanely lucrative, but best served with a female POV character who beats you over the head with feminine takes on sensation and emotion. If you can pull that off, you're a better woman than me. Can theoretically be done with a male protagonist, but men mostly just watch free porn instead.
6. Thrillers, Horror, and Fantasy!
This is totally the way to go. Especially with horror and fantasy, where you don't even really need to know history or how the real world works. As long as your story world is inherently consistent and believably logical.
And then you've got to consider...
Adult, Young Adult, or Middle-Grade/Early Reader?
I would avoid writing for an adult audience at all costs, unless you are in an adult-specific genre, or just feel you have to follow your muse rather than prostituting yourself. Adults read far fewer works of fiction, demand a freshness of plot that younger audiences don't yet know to demand, get annoyed with long series of books, and expect a book to be like 400+ pages. All of which is crazy.
YA is absolutely dominated by chick lit. Something about that broody, teenage chick mentality just drives it to endless consumption of supernatural romance. A lot of boys get less lit-focused around this age, having gobbled up their Harry Potter, R.L. Stine, etc., closer to 12-14.
Middle grade is the plum market for the male writer who wants to work in Horror or Fantasy. Just no smut, and no really graphic violence. MG boys are the quintessential Starving Crowd of the bookstores. The shelves literally burst with 10, 20, even 50 book series with covers that say things like, "5 Million Copies Sold!" for even mediocre titles. And the market renews itself yearly.

The number of people working this model in the self-publishing world is staggering. Which is great, since fiction is a cooperative market. Plus, length expectation is only 30,000-50,000 words. You can get away with going longer, but that'll limit the number of titles you can crank out. Since nothing I write will ever get the kind of traction a Harry Potter series did (in no small part because I don't write that well), it will be far better for me to concentrate on more, shorter titles if I want that island nation while I'm still young enough to enjoy it. (An added bonus: these titles don't require a $1,000 investment in some piece of Boris Vallejo quality fantasy art for a cover. A cartooney cover works just fine.)
**********
So the upshot for me: Middle Grade Fantasy. Think shorter, more formulaic Harry Potter or Percy Jackson. Or longer, less formulaic Goosebumps. Or similarly-sized, slightly different Ranger's Apprentice or The Last Apprentice with a different D&D character as the young protagonist. These last books are serving as the plot templates for this first effort.
Done so far: Two prologues (really the same prologue, written twice), focusing on the shadowy introduction of the Big Bad, out of which I chose the one with the voice I liked better; and also chapters one and two, wherein our hero is disappointed to see the world as he knows it thrown into upheaval, and is set upon his coming of age journey. Will be slightly lighter and more comic in tone than the other
Apprentice series books.
Got myself a little plot. Got myself a little protagonist. Nice little conflict brewing, there. Some hurdles to overcome. Etc.
Ain't gonna win any Pulitzers, but I'll be amazed if they don't sell. The recipe for success in self-publishing these days is pretty well worked out, and doesn't often fail unless you simply run out of steam before you finish writing. Naysayers are of course welcome, but I think you'll be surprised by how easy it is to make a buck in this gig.