Looking back there are parallels between Roland and Angela (Library of Ruina) and The player and Mychael, coming from separate worlds,
knowing of their own world, teaching each other about their own respective worlds (Roland teaching Angela about the city, and Angela teaching Roland about the Library, Me teaching Mychael about the wider world, and Mychael teaching Me of his)
The outside character (Roland and I) being drawn to the other world by chance
And several more spoilery stuff