Book marketing is not a monolithic thing, it doesn't happen at one single time, and your marketing activities will change over your career as a writer.
There is no magic bullet, only a number of different options that you can utilize along your author journey to craft the best plan for your book. 
Here are some of the opposite polarities on the marketing scale that, along with your definition of success, will help shape your marketing activities.
You will move up and down on these scales with every book and as you change over time. There is no ‘right' way, just a series of choices that can result in multiple combinations of book marketing approaches.
This is an excerpt from How to Market a Book Third Edition, available now in ebook, print and audiobook formats.
Short term vs. long term
Many new authors with only one book will focus on short-term sales spikes because they can't yet imagine a future with more books. I was exactly the same! Short-term sales are fantastic for that initial launch push, but they often cost money and are not sustainable over time. After all, once you hit the top of the charts, there's only one way to go from there.
If you want a long-term career as an author, you also need to think about long-term marketing. You want to build a sustainable baseline income, money that comes in from your books consistently every month. This comes from having more books on the market but also from building your email list and other platform-related marketing activities.
Focused vs. eclectic
Some authors focus entirely on one brand, one genre and one type of marketing. They go deep and become an expert, ignoring everything else in their strategic direction.
I wish I could be like that sometimes! But the opposite is someone like me who likes to write in multiple genres, who has a bit of a magpie brain and likes to try different things over time, who can't focus on just one way of marketing.
Both can be successful, and because it often does come down to personality, it's important to avoid comparisonitis with other authors who do things differently. See my book The Successful Author Mindset if you are struggling with this or other mindset issues.
Income vs. brand building
Some marketing activities are about making direct income, whereas others are about brand building. For example, getting on a network TV show or a national radio show can be amazing for building awareness of your author name, and as social proof for your website. Having a physical book launch party at a swanky location might make the local paper, or give you some great photos for social media.
But if you want to make direct income, you'd be better off publishing a multi-book-boxset and using some paid ads to direct sales there.
Paid vs. free (or money vs. time)
I've spent nearly ten years building up an author platform using my time, not my money. It costs little to set up a professional website with an off-the-shelf theme, and it's free to write articles, or podcast, make videos and share on social media. It's free to self-publish on all the major platforms, and you can find out everything you need to know from free and cheap books and resources. You can definitely build an author platform and generate good sales from free marketing – but you will pay with your time.
If you are short on time and want to pay for marketing, you can spend whatever you like,