More like Gregor the Overlander

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Underland ChroniclesSeries
series5 books2003
Fantasy and epic fantasy, adventurous in tone, about coming of age and quests, set in underground & caves.
“The Underland is inhabited by humans who traveled below hundreds of years ago, along with various giant versions of creatures like bats, cockroaches, and rats.”
from the catalogue
Start with Gregor the Overlander →
NarniaUniverse
universe38 books1950
Set in secondary world.
“Once there is a wardrobe at the back of a spare room, and behind the fur coats the wood gives way to snow, and a lamp-post burns alone among the trees.”
from the catalogue
Start with The Lion, the Witch and the Wardrobe →
includes Narnia, Narnia Solo Adventures
CoralineSeries
series2 books2002
Horror and gothic, eerie in tone, set in parallel universe.
“The Guardian ranked Coraline #82 in its list of 100 Best Books of the 21st Century.”
from the catalogue
Start with Coraline →
Un Lun DunNovel
China Miévillestandalone novel2007
Whimsical in tone, about chosen one, set in otherworld and big city.
“The title is derived from 'UnLondon,' the name of the alternate realm where the book is set.”
from the catalogue
SummerlandNovel
Michael Chabonstandalone novel2002
Whimsical in tone, set in parallel universe.
“Summerland weaves elements of a World Series, parallel-universe road trip, and a hero's odyssey.”
from the catalogue
The Paradise TrapNovel
Catherine Jinksstandalone novel2011
About survival, set in otherworld.
“Eleven-year-old Marcus is spending a week at the beach with his mother in a tiny old travel trailer, but boredom turns to terror when a new friend discovers a cellar that has magical doors through which people keep disappearing.”
from the catalogue