Marketing is great (and it really does help), but how good is your book?
Can you stake your life on your writing?
I usually recommend that people cut their teeth on short stories before migrating to long-form writing. Short stories are harder to write (irrespective of what anyone thinks). Going this route teaches discipline with words. You'll learn proper structuring. You'll learn how to tie loose ends and look out for all characters. It's a by-force learning curve because with short stories most errors are glaring.
Reason why I recommend this is that a lot of people become famous from penning down short stuff in magazines (Case Study: Leslie Arimah. Although she did win a competition, too). People would have seen snatches of what you can do and more often than not, they'll line up whenever you decide to release a book.
Of course this doesn't work for everyone. Some write short stories and never go anywhere. Some write one book and make it. Hurrah!
What I'd advise is get a copy of your book and give it to a critic. Not a bad belle person (like me)... but someone who will tell if he would buy it and not mince words with you. Every exceptional writer I know has one of these. If the person says your book isn't good, take it off, rework it and put it back.
P.S Try to stay away from family and close friends. More often than not they'll lie through their teeth that you're good even if you're not.
If you know your book is awesome and you're sure, market away, please. See if you can get it reviewed on Okadabooks. Try to get influencers (maybe, Ofili) to read and talk about it. Add long comments like mine on writing forums...sell yourself (subtly). People will notice you naturally and want to see what you've written.
All these things are free. No money spent. Just time and lots of sincere human interaction.
How about that for starters?