Question

Why do books pages turn yellow or brown when done reading them?​

Answer 1

Books that are surrounded by oxygen will turn yellow, so basically, all books since oxygen is constantly surrounding us. Oxygen-rich conditions are very bad for paper, which gives them the yellow tiny. This is actually called lignin oxidation! Lignin oxidation gives it a yellow or brown colour as you said. Yellow comes first, then brown after.

I hope this helped a bit! :)