More like The Thirteen Secrets

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The Dark Is RisingSeries
series5 books1965
Mythic fantasy.
“The first book in the series, Over Sea, Under Stone, was originally conceived as a stand-alone novel, and the sequence gets its name from the second novel in the series, The Dark Is Rising.”
from the catalogue
Start with Over Sea, Under Stone →
ChrestomanciSeries
series12 books1977
completed
Whimsical in tone, set in parallel universe.
“Greenwillow Books published a US edition within the calendar year.”
from the catalogue
Start with Charmed Life →
AlderleySeries
series4 books1960
Mythic fantasy, about quests.
“The three books span most of Garner's writing life rather than forming one continuous trilogy.”
from the catalogue
Start with The Weirdstone of Brisingamen →
Endymion SpringNovel
Matthew Skeltonstandalone novel2006
About books and reading.
Five Children UniverseSeries
series27 books1902
completed
Set in early 20th century.
“Neither book is by Nesbit herself, and the two have no direct connection to each other beyond both drawing on her creation.”
from the catalogue
Start with Five Children and It →
Charlie Bone UniverseSeries
series14 books2002
Fantasy, adventurous in tone, about magic schools, set in british isles and contemporary.
Start with Midnight for Charlie Bone →