What can we conclude?

The children's author does have the advantage of a loyal and enthusiastic readership who are keen to gobble up the next installment. I think radio plays with their daily broadcast and cliffhangers are a thing of the past. Those who do read the paper are happy to see their Mandrake snipped into little pieces or keep collecting comic books as they're 'just getting into this story'. The Marvels and DCs were a mix of 'to be continued' and self-contained stories so they may have jokingly hyped all other aspects but fidelity to the story was still paramount.

Books of the same series share this last split between stories that carry over and books that contain the same set of characters but in different adventures.

Harry Potter is more or less contained but, by not being reliant on a single author, Nancy Drew can continue; her adventures started in 1930.

With series we must also take into account the difference between over 300 installments of the Berenstain Bears selling 260 million copies to just seven Harry Potter books and eight supplements at the top selling 500 million copies and five Robert Langdon books by Dan Brown in equal seventh place shifting 200 million copies.

Also bear in mind (not a Berenstain bear though) the Langdon books began their publishing history in 2000. Fifty Shades is even more recent, there are but three books in the series published between 2011-2015 and they've collectively sold 150 million copies, putting them in the company of Little Critter (over 200 installments) and Peter Rabbit (6 installments)

Children's books feature heavily in the all-time bestseller series list. Jeff Kinney's twelve installments of Diary of a Wimpy Kid have sold 194 million copies and then there's the Baby-Sitter's Club which is aimed at girls age 11-13. Three hundred and thirty five installments since 1986 ensure that there is a steady supply of Ann M. Martins and Baby-Sitter's Club readers. 172 million of them.
Though if you did want to dwell on the statistical aspects of these calculations, this series, along with the others, has to divide total sales by number of installments and then consider that it is most likely the same readers that bought the first Geronimo Stilton in 2000 who are buying the latest one now, or at least there's significant overlap. This doesn't matter to the publisher, the author, the agent, or the reader when the profit margins are so high and when it's an easier launch than one for a book by a new author on an unfamiliar or not immediately appealing topic or in a less readily accessible genre.

At least, it doesn't matter unless someone on that chain is a bit cutthroat and wants a blueprint for the future in case things change. What is the appealing aspect of this series by Elisabetta Dami? How can it be distilled, tweaked, co-opted?

In a rare case of back adaptation, over three hundred Star Wars books have been written by various authors since 1977 and they've collectively sold some 160 million copies.

And, though it's heading further out into wild theorising, it's possible that Perry Mason's cases were documented in the forty years where other media were just coming on line. There are now so many cases to crack on cable. There again, it remains a popular genre in paperback and hardcover.