We've all experienced trilogies, quadrilogies and even longer series of novels. Some authors only ever write standalone individual books, others love to write larger stories - sometimes a LOT bigger.
But what do readers like?
There's a lot of advantages for an author, especially an indie author, in writing a series as it can really help with the marketing side of things. It allows the author to pour most of their marketing effort into the start of the series in the sensible hope that most readers of the first novel will go on to buy some or all of the rest of the series. Obviously that relies on a good first novel.
The big weakness with that plan is that your first novel is most probably the worst one you'll ever write. Book 3 will probably be a much better novel as you will have honed and polished your writing skills, but readers prefer not to enter a series part way through. If your first novel is good, you are on safe ground, but if the first in the series is the weak link it can kill sales of all the others.
So what's the solution? Some people recommend writing two or three novels and just throwing them away, but I would never advise that. Your first novels are not only necessary to improve your creative skills but also to sort out the skills and help needed to edit and finish your novels. Attempts to promote those novels will also teach you a huge amount about the market, advertising, sales blurb, making contacts and how to use social media to promote your brand. You're going to make mistakes, probably a lot of mistakes, and it's much better to make those mistakes on your early books.
Save your best efforts, once you've learned some of the tricks of the trade and how to avoid the many pitfalls, for your later books. This is a circular thing as the tips you learn from attempting to market your first couple of books can also help you plan and write your later books. Whilst it's nice to think of writing something new within your genre, you also need to not stray too far from what readers in your genre expect. Feedback can be especially important here, and you won't get much feedback if you don't publish anything.
So how does this all work together?
Well the detail is very much up to you, but I would advise writing a couple of standalone novels first. It's the best way to learn and they will be most helpful to practice your marketing skills, building a following and making some useful contacts. With all you learn from that experience, crack on with a series.
Some genres are better suited to a series of novels than others, but within Fantasy, Science Fiction and Thrillers, a series will usually be well received by readers and you will benefit from increased sales.
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