The book you're looking for is The Little Schemer (just noticed another commenter suggested this as well). Even if you're not that interested in LISP, you'll co

The book you're looking for is The Little Schemer (just noticed another commenter suggested this as well). Even if you're not that interested in LISP, you'll come away from this book with a solid grasp of "programs as recursive functions." I found that the format of the book lends itself well to reading during a commute. It's easy to pick up and work through a few pages. If you get to a section that you don't understand right away, slow down and make sure you get it before moving forward. https://mitpress.mit.edu/books/little-schemer https://mitpress.mit.edu/books/little-schemer