Goodreads Librarians Group discussion
Serieses!
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Whitman books page?
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To be a series, books need not have ISBNs (or ASINs, for that matter). Books published before the 1960s can be in series.
However, they do need to have common characters, etc. It doesn't seem like the books you are talking about have that.
However, they do need to have common characters, etc. It doesn't seem like the books you are talking about have that.

I’m talking about the Whitman picture books from the 40s & 50s, for example.