The Divide - Elizabeth Kay

Felix is not your usual hero. Though brave and intelligent, he suffers
from a rare illness. His parents take him on the holiday of
a lifetime to Costa Rica, where they visit the high point where
the waters that feed into the Atlantic and the Pacific Oceans separate.
There, Felix faints and something extraordinary happens.
He wakes up in a back-to-front world where magic is real and humans
are imaginary. Luckily, he's taken under the wing of Ironclaw,
a maths-loving Brazzle, anda mischievous tangle-girl called Betony.
Together they embark on the wildest adventure.