Structure and Intepretation of Computer Programs vs The C Programming Language (AKA: K&R)

This isn't a book you'll glean direct practical tidbits out of, an introduction to not only functional programming but how to think in a paradigm outside the usual way C*/Python/Ruby/Java/etc... are coded.

Even if you wind up never working in Scheme or any other primarily functional language, the tactics and thought processes you'll learn here will apply to any currently-evolving language to a greater or lesser degree. You'll be able to map your thought process into the paradigm that works best for your current situation and not just be forced into a limited set of idioms that causes unnecessary boilerplate and clunky code mangling.

Like closures and encapsulation without language support of objects.

This book gives a great insight about immutable and mutable state (with pros and cons), typisation, FP, OOP, and many other things in a very beginner-friendly manner

This book is so well known that it's affectionately called "K & R", after the authors.
It's been cited in many other books and is familiar to most, if not all, CS students.

This book is written by Dennis Ritchie, who was one of the main people behind the development and design of C and UNIX.

The challenges at the end of each chapter do a great job of requiring many of the skills learned up to that point. Completing the challenges is a great way of insuring you understand the material.