Suggest me a horror-esk book where the main character goes deeper and deeper into ‘the unknown’.

Something sort of like the Left-Right game, where the main character leaves our/their normal world behind and embarks on a journey into surreal/strange/unsettling lands unknown- something where they go deeper and deeper into a world that wasn't meant for them, or that grows stranger and stranger until there's no turning back. by The_Medic247