To Series or Not to Series
It has been an axiom of publishing as long as I can remember that when creating a book, consider it being part of a series. If a reader likes one book, they will possibly like and buy others in the series. This makes the marketing and promotion easier (and more cost-effective). It is also easier on acquisitions and development costs, being that concept formulation, design, and author interactions becomes more stream-lined and efficient.
There are many instances when a series is the right call. Whether it be instructional books or information intensive books (like the Dummies Guides) or a universe-building fiction series, there are some great applications for the concept.
There may be times, however, when the series idea is misapplied. Some writers may take their fiction idea and broadcast it over a series of books. They may be, gasp, padding it to have it reach three or four books. In some of these cases, stories or books may be better served by being tighter narrative by being a single or fewer volumes. Getting feedback from ideal readers may help guide you here.
As for non-fiction, think of the reader interest in each installment in the series. Are they all of equal interest, or are some "A grade" and others also rans? In this case, better to combine titles or drop tertiary ones. Once again, potential customer feedback from a quick survey may help flesh this out.
Do not shy away from publishing in the series format. But do so for the right reason. Market feedback and your P&L should dictate your decisions. Reach out to me to discuss your publishing ideas. Maybe in a series of calls.