A Phantom Series is one that is advertised but never published. Altemus advertised a boy’s series called the Ben Lightbody Series in the 1912-1914 era. The series was noted in numerous book and print ads. For whatever reason it was never published.
I have just come across another phantom series. It is mentioned in a periodical called the Critic. The excerpt below comes from the June 19, 1897 issue.
There is no evidence that the “American Series” was ever published. Trif and Trixy was published in 1897. Of great interest is that although many copies (in red and green) show the 1897 date at the base of the title page, there is a way to distinguish the very rare first edition from the later edition.
Look carefully at the page below which is the last page of ads in the back of the book. The earlier 1897 book lists Trif and Trixy at 35 cents whereas the later lists it at 50 cents. Subtle but real. Of course, there is no evidence it was offered at 30 cents which is the phantom series price.
For completeness copies of the books are shown below.