The little schemer book https://mitpress.mit.edu/books/little-schemer-fourth-edition is my favourite introduction to scheme and to functional programming.
Fair warning: Don't be fooled by its cute images. This is a difficult book. It needs heavy work from you to understand; I've done some analysis on racket in this gh repo for anyboddy interested: https://github.com/spapas/little-schemer
Unfortunately, I was never able to understand the Y combinator chapter :(