A story that was published in two or more parts in a magazine. Sometimes these parts were collected and published as a book, occasionally part of a series.
For people who are generally outside the series book collecting field, the phrase “serial” might be applied when “series book” is intended.
The image below is a sample page from American Boy magazine containing a story that was later expanded and published as a volume in the Adventures in the Unknown series (Grosset & Dunlap, 1933-34) by Carl H. Claudy. The illustrations in the book were different as well.
When published in the magazine, it was a serial story, and then it was published as a book. This is different than a fixup where a series of short stories are turned into a book by connecting them as if they were one story.
This definition and discussion of a term applied to collecting juvenile series books is part of a Glossary of specialized terms. At the outset a new term or topic is added once a week on “Terminology Tuesdays” on Stratemeyer.org.