Jag tror att det här är Sten och himmel av Graham Edwards.
The book, as well as its sequels, follows the adventures of British historian and naturalist Jonah Lightfoot, who is caught in the eruption of Krakatoa in 1883. The blast transports him and American runaway Annie West into a vertical world consisting of a seemingly infinite wall populated by crumbling civilisations, weird creatures, and sentient dragons. No one knows where the wall begins or ends, and no one dares to climb to its top or fall to its base.
This world is called Amara, and it is a place deeply entwined with our own world. Throughout the books Jonah and his companions traverse the world and uncover its many mysteries. The true nature of Amara is fully revealed in the second book of the trilogy, Stone and Sea.