Writing like any business, needs investment. For those who independently publish, they invest money in editing, covers etc, but that's it. For traditionally published authors, we often wait for the book to be published then sit back and hope for the best or make use of free marketing.
If you can afford it, you need to consider investing money in your books. They were worth your time, so they should be worth your money. When I started writing, I made very little money so I set a budget of 10% of my income. Now this comes to a nice sum that I can invest in advertising opportunities.
~Set your budget and don't go over it.
~Find out where you audience is. If they're on a forum, take a look at their advertising options to see what the cost may be. I don't recommend paying for Facebook ads because even though they allow some targeting, the targeting is too wide and you're not hitting the book lovers. (More on this next week) Consider book email lists like ones run by Bookbub or Digital Book Today. You will be directly hitting the inboxes of genuine readers who want to buy a book.
~Make use of your audience. Hopefully by now you have an audience on Facebook or your blog. Let them help you find more readers. Offer a giveaway (a $2.99 book of their choice is a cheap incentive) for spreading the word. The likelihood is that if they are readers, they have reader friends.
~Don't overspend. It can be easy to get carried away with the paid opportunities (another reason to avoid Facebook ads. It's easy to keep adding money to your budget). Be realistic and make the most of what you can afford.