Whitman published both series, starting in the 1930s. My (so far) limited research tells me that Better Little Books came second, and possibly as a continuation of Big Little Books, i.e., the name of the series simply changed from "Big" to "Better" - and supposedly the numbering continued without a break. I have some Better Little Books, but they seem to have been publsihed while the Big Little Books were still being published, not later. For example, I have a Better Little Book #1436, "Maximo the Amazing Super-Man" published in 1936. However, the db already has a Big Little Book #1436 and it's "Jack Armstrong and the Ivory Treasure."
A review of the books in the db seems to indicate that a Big Little Book is assigned its issue number (e.g., 1436), and a Better Little Book is assigned its issue number with the /A addition. I'd like to get confirmation of this before I add my Better Little Books into the Big Little Book title in the db.
r/Mike