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.

Superior series sear themselves

When it comes to series, it's instructive to see most popular by genre but these are the biggest sellers

Title	Author
Harry Potter	J. K. Rowling
Goosebumps	R. L. Stine
Perry Mason	Erle Stanley Gardner
Berenstain Bears	Stan and Jan Berenstain
Choose Your Own Adventure	various authors
Sweet Valley High	Francine Pascal and ghostwriters
Noddy	Enid Blyton
Nancy Drew	various authors as Carolyn Keene
The Railway Series	Rev. W. Awdry, Christopher Awdry
San-Antonio	Frédéric Dard

Where can I find a literary agent?

For a poet this question is more pertinent than to other bloggers who confine their writing to online fora. I have entered a few competitions since I stopped work but I haven't propelled my skills into the worlds of enterprise and commerce, despite my efforts.

That aside, I wanted to know the most successful literary agents. And then on the same Writing Tip Oasis I found a list of top Australian literary agencies so I'll be bookmarking that.
The Directory of Literary Agents wisely make their determination on the best by looking at the categories in which they excel. While a longer list suggests that there are inclusions and rankings that help tell the story, not available to the frustrated author who sticks to the top ten:

• Robert Gottlieb (Trident Media Group)
• Marly Rusoff (Marly Russoff & Associates)
• Russell Galan (Scovil Galan Ghosh Literary Agency)
• Kimberly Whalen (The Whalen Agency)
• Celeste Fine (Sterling Lord Literistic)
• Jenny Bent (The Bent Agency)
• Scott Miller (Trident Media Group)
• Larry Weissman (Larry Weissman Literary)
• Frank Weimann (Folio Literary Management)
• Stephen Hanselman (LevelFiveMedia) 
• Daniel Lazar (Writers House) - since Trident Media Group picks up two mentions

If I was tasked with compiling a list from scratch, I would look at things like the advance paid to an author. From a publisher's perspective, one might be kindly disposed to a proposal from Carlo Collodi's agent since Le avventure di Pinocchio. Storia di un burattino has sold more than 80 million copies.

This is not to downplay the fact that some literary agents are also going to be a better suit to someone writing about welders and others to the author of a book whose plot revolves around the hay shortages during the drought.
We might look to a literary agent who has connections to this publisher who like to feature claypans in their books and that's what you can write well on.
I suspect though that it would be easier to use the broad categories; picking Mr Gottlieb for his expertise in promoting books in the popular 'fiction: thriller' category.

Book early

You know how Wikipedia can be updated just like that because of its multiple contributors, well they've sideswiped two or three of my posts by now putting the Napoleon Hill classic in tenth spot. It wasn't there before, not by a long chalk. Biggest non-fiction, yes.

Let's move on. It's a lesson in the mutability of online text.

It appears there's this whopping big oral tradition before bound parchment and that's what all those epics were about, along with the free borrowing from earlier tales. The earliest extant books make for a fascinating read.
There were quite the ramifications when they discovered the Dead Sea Scrolls

Book doubt

There's no question that books still hold a special place. Imagine trying to get the blood pumping over the biggest seller of steel girders or marveling at the amount of floor space devoted to the sale of air compressors?

It has the been the long contention of tyrants that books can have a deleterious effect. Indeed it is the plot of one famous work Fahrenheit 451

And occasionally an obsession with books can be fatal though you would have to be very unlucky.

II

There are a couple of things to note about the ultimate bestsellers.  Yes, fantasy dominates and no doubt if you traveled further down the list you would find plenty of novels that fit that description. Those fellows who've dabbled in literary studies will bristle at each mention of the novel Don Quixote since it is actually a romance.
You really get your HECS money's worth when you find yourself in an exclusive club that doesn't then think of Barbara Cartland.  I don't blame people with only a passing interest in English as she is spoke to know the long works written in the seventeenth century and earlier, and what they were called.

So now your top ten novels must include The Lion, the Witch and the Wardrobe and let's add She by H. Rider Haggard for effect.

What other romances were there of note? I'm imagining Gargantua and Pantagruel
"Unlike the later form of the novel and like the chansons de geste, the genre of romance dealt with traditional themes. These were distinguished from earlier epics by heavy use of marvelous events, the elements of love, and the frequent use of a web of interwoven stories, rather than a simple plot unfolding about a main character. The earliest forms were invariably in verse, but the 15th century saw many in prose, often retelling the old, rhymed versions." [Huizinga, Johan (1996). The Autumn of the Middle Ages. Chicago: University of Chicago Press. p. 354.]

From attic to seller

That chapter two was a list of biggest booksellers apparently, though I wasn't aware that sports goods stores sold that many books, never mind businesses with Archery in their name. Perhaps I've struck on a bum list but Barnes and Noble are there.

I like Bustle's list of world's largest bookshops

1. Powell's Book Store
2. Barnes and Noble in Union Square
3. Cook & Book
4. John K. King Used and Rare Books
5. Waterstones Picadilly
6. The Strand
7. The Bookworm
8. Bart's Books
9. El Ateneo Grand Splendid
10. The House of Books

but, really, you can see why they've also included Blackwell's (the underground Norrington Room is 10,000 square feet and holds the record for largest single room selling books) and Librairie Avant-Garde