You have a standard marketing challenge. You should probably read up a bit on basic marketing strategy.
You have two choices, spend money to market or spend your own time (and skills). There are LOTS of choices.
Usually you start by understanding who is most likely to buy your product and then trying to understand their habits so that you can get yourself in front of them somehow.
You also need to understand your business model. You need to construct a price point for the product (your book) which exceeds the expense of selling it. Much of this depends on volume of sales, which is something you can't predict easily. It's a challenging process.