Keeps going, and going, and going...
These are the literal never ending stories. How many books does it take to start and finish a good story? Often times it only takes 1 book. A lot of the time it takes 3. And sometimes it takes 18 or more, plus several side stories.
Authors sometimes can't get the story out of their heads and need to share more like Marie Lu's return to the Legend series with her newest addition Rebel. Or all the side characters and adventures spinning off of Leigh Bardugo's Grishaverse. Or even the start to the world of Panem in Suzanne Collin's anticipated prequel to The Hunger Games, The Ballad of Songbirds and Snakes.
So while most of the longest series in YA listed below have officially finished... you never know. There still might be more to tell.
The Selection Series
The Selection by Kiera Cass
America Singer is chosen to compete in the Selection--a contest to see which girl can win the heart of Illea's prince--but all she really wants is a chance for a future with her secret love, Aspen, who is a caste below her.
Shatter Me Series
Shatter Me by Tahereh Mafi
Ostracized or incarcerated her whole life, seventeen-year-old Juliette is freed on the condition that she use her horrific abilities in support of The Reestablishment, a postapocalyptic dictatorship, but Adam, the only person ever to show her affection, offers hope of a better future.
The Shadowhunter Chronicles
City of Bones by Cassandra Clare
The Shadowhunter Chronicles, also known as Shadowhunters, is comprised so far of three series (twelve novels as of 2019) and several short stories written by American author Cassandra Clare, along with some companion books, a graphic novel, manga, film, and TV series adaptations.
Each of the stories follow the supernatural conflict between the Shadowhunters, a group of powerful human-angel hybrids, and Downworlders, a society of mythological beings, and their battle against demons. The Infernal Devices series is the prequel to The Mortal Instruments series.
The reading order below is suggested by the author.
I Am Number Four by Pittacus Lore
The series follow the story of nine human-looking aliens brought to Earth when they were six years old. Their purpose is to grow into their powers and return to their planet, Lorien, and get it back from the evil Mogadorians. The teenagers have been protected by a charm, which only allows them to be killed in a set order, and by non-gifted guardians called Cêpan.
Up to the beginning of the first book, six have survived and reached 15 years. As their powers, their Legacies, start to develop they can't stay hidden anymore.
The Vampire Diaries
The Vampire Diaries by L. J. Smith
Presents the first two volumes in the Vampire Diaries series, in which popular and beautiful high school senior Elena finds herself torn between two vampire brothers, the dark and brooding Stefan who wants to protect her from the horrors of his past and the dangerously attractive Damon, who is determined to use her to get revenge on his brother.
Pretty Little Liars
Pretty Little Liars by Sara Shepard
When one of their tightly-knit group mysteriously disappears, four high school girls find their friendship difficult to maintain when they begin receiving taunting messages from someone who seems to know everything about their past and present secrets.