As part of the MOOC course Paradigms of Computer Programming (https://www.edx.org/course/louvainx/louvainx-louv1-01x-parad...) I read the initial parts of the book: "Concepts, Techniques, and Models of Computer Programming" (mitpress.mit.edu/books/concepts-techniques-and-models-computer-programming). The initial chapters provide a good approachable way to get started with functional programming. Though working with Oz (the programming language used in the book) gets some time to get used to, this is an excellent book. I am bit surprised that this book is not as well known, and probably I wouldn't have known about this book if not for the MOOC course.