Publishing “out of series” means a book does not fall within its publisher’s regular series numbering system. It can happen for various reasons, such as being part of a spin-off series, prequel, sequel, etc.
There are various methods by which a book can be published outside a series. One way is for an author to create a stand-alone novel and then later decide to turn it into part of a series; when this happens, subsequent books in that series can then be released simultaneously with earlier editions in its series. It may occur when an author writes something popular that they did not initially plan to turn into part of their series.
An author may publish their book out of the series if it forms part of an ongoing series but later ends due to cancellation or discontinuity. When this happens, the book becomes stand-alone since there no longer is any need to fit it into a series.
Sometimes, an author or publisher may decide to publish a book out of a series for various reasons. One reason could be that the novel fits differently with its series’ continuity – for instance, if its setting or genre are different; another could be that they believe its success would be more significant when published separately than as part of an ongoing storyline.
Out-of-series books are integral to publishing, giving readers more options to select books. Furthermore, out-of-series titles allow authors to explore unfamiliar genres or subjects – ultimately leading to more successful books that become popular and widely read.