Your first book in my opinion should be a series. I wrote four books before I started editing the first book. Although, some may say that it’s a bad idea to do because your audience that has read your first book can give you useful tips and insights on character development or any small aspects of your book for the next in the series.
However, I already knew where I was taking my book when I was writing it. I had a vision and I stuck with it. I might’ve had some changes but those were made because I placed the book down for a few days came back with a fresh take on it. Besides I was writing because I wanted to put my ideas down somewhere I wasn’t thinking about publishing my book until I had an entire book written and now I have four in the works.
Having a series will help with promotions and keeping the story alive. I’ve read books where series were released years apart and I had to reread the first book to remember the story. Although rereading was great, I found that at least 6-7 months in between books is ideal to keep the story fresh in your mind and the reader eager for the next.
Also completing the series doesn’t mean that you can’t go back and revise the story. It only means that you’re ahead of the game. This leaves you to start on another book or another project or to focus on advertising book 1 because you know that book 2 in the series is complete.