Reading Order List tracks the reading order of more than 26,000 book series — 676,000 books by 18,000 authors, updated daily. That database turns out to be a decent telescope for watching publishing itself change. We ran the numbers on every multi-book series we track, and the picture is unambiguous: the book series is shrinking, and it is speeding up.
Series are a third of the length they used to be
Among series that are finished — so we're not unfairly comparing against sagas still in progress — the average series that began in the 1950s ran to about 14 books. For series begun in the 1980s it was 9. For the 2010s: just 5 books.
The median tells the same story more bluntly: half of all completed series started since 2000 wrapped up in 4 books or fewer.
Some of that older average was carried by the great mid-century paperback engines — but even setting those aside, the direction of every decade since 1950 is down, without a single reversal.
The trilogy is taking over
In our data, the share of completed series that are exactly three books long has doubled — from 15% of series started in the 1950s to nearly 30% of series started in the 2010s. The trilogy is now the default shape of a book series, especially in fantasy, science fiction, and romance, where publishers and indie authors alike plan three-book arcs from day one.
Duologies are quietly rising too. Add them together and roughly four in ten modern series are done in two or three volumes.
Trilogies used to take 17 years. Now they take 2.
The most dramatic number in the entire study: the average time from a trilogy's first book to its last has collapsed from 17.5 years for trilogies begun in the 1960s to 2.3 years for those begun in the 2010s.
Two forces drove this. Traditional publishing tightened series scheduling — a book a year is now standard contract cadence. And the indie rapid-release movement went much further: many self-published authors write an entire trilogy before launching book one, then release all three within months to satisfy binge-readers and retail algorithms.
Whatever the cause, a reader starting a new trilogy today can reasonably expect to finish the story. A reader in 1965 was signing up for a decade-and-a-half relationship.
The never-ending series is real — and recent
Of all series started in the 1990s, about 7% are still marked ongoing in our database today. For the 2000s it's 8%, and for the 2010s, 12% — series old enough that readers have been waiting a decade or more for the next installment. (We track 7,700 ongoing series in total; more than half of everything started in the 2020s is, reasonably, still open.)
Every reader can name a famous example of the genre-fiction cliffhanger that never resolved. The data says they are not imagining it: the longer a series runs past its expected length, the less likely it is to ever be marked complete.
The longest series ever published
Setting aside publisher lines like Little Golden Books (754 titles across 190 years), the longest true series in our database are the great action-paperback machines:
The Executioner by Don Pendleton — 460 books (1969–2020). Longarm by Tabor Evans — 436 books. The Trailsman — 390 books. Nick Carter: Killmaster — 264 books. Among living franchises, Warhammer 40,000 and The Horus Heresy together account for more than 460 volumes and are still growing.
These were largely house-name series — teams of writers publishing monthly under one byline for decades, a model that modern collaborative indie universes have quietly reinvented.
The new series machines
The fastest series producer in our database is Michael Anderle, with 1,881 tracked titles since 2015 — roughly 170 books a year across his collaborative universes, a pace the Longarm ghostwriting stable of the 1980s would have respected. The rapid-release model hasn't just shortened series; it has industrialized them.
What this means for readers
The practical upshot: starting a series in 2026 is a much smaller commitment than it was for your parents. Most new series will be done in five books or fewer, and a new trilogy will likely be complete within three years. The flip side is fragmentation — more series than ever are being started (we logged over 10,000 new series beginning in the 2010s alone, double the 2000s), so choosing what to read next has never been harder. That, conveniently, is what this site is for: find any series and read it in the right order.
Methodology
Numbers come from the Reading Order List database of 676,524 books, 18,352 authors, and 26,479 distinct series, compiled from public book databases and updated daily, as of September 2026. Length comparisons use only series marked complete or likely complete, so ongoing sagas don't drag down recent averages. Catch-all shelving categories (anthologies, collections, standalone groupings) were excluded. Book counts include all tracked entries in a series, which for some franchises includes novellas and companion volumes. Historical decades are subject to survivorship: older series that nobody reads anymore are less likely to appear in our data, which if anything means older series were even longer than reported here. Feel free to cite these figures with a link to this page; for questions or the underlying queries, contact us via the site.