it piqued my interest too. i haven't been able to find anything conclusive, but the note in the corner, "Dreamstalker's Textbook", did lead me to this book (series?): https://www.goodreads.com/book/show/206517820-the-night-train

it's apparently about a world where people don't dream anymore and nightmares have become real, which could match the "two worlds coming together" thing.

in a review of it on another site, someone mentioned there are illustrations at the start of each chapter, could be the source of them.

I did come across that series while I was looking, it's for 8-12 year olds and the art style from the back of the book doesn't look right.
Maybe it's from a web comic ?