In a series of any length, many publishers and authors find it advantageous to promote their previous and future titles so that readers may look for them.
A future title in a series can be announced in an advertisement or at the end of the present story with some phrase like “little did the Hardy’s know that they would soon be involved in their next mystery …”
Sometimes the next title is published but different from the advertised “phantom title.” The reasons for this can be many. A title proposed may be too similar to others in the series or books by other authors. An event like a war starting or ending might change plans for including the character in the larger world events.
Most often the phantom title is proposed at the end of a series. The publisher may have decided that the sales of the existing volumes did not merit issuing a new volume.
For collectors, seeing reference to titles that were not published causes them to be added to “want lists” and create the illusion to others that the books were actually published but are merely very hard to find. This is particularly the case for a very scarce “series” of one published volume like the Stratemeyer Syndicate’s White Ribbon Boys (Cupples & Leon, 1916). The advertised second volume was never written or published.