Learning a new concept is naturally easy when it's conjoined with something familiar. This book employs this method by associating new interval sounds with familiar song sounds.
You will learn how to name any interval you hear and sing any interval from memory. This is the study of relative pitch, which is an indispensable asset to your musicianship.
You will find it easier to play songs by ear as you practice recognizing and singing intervals from melodies and chords. You will also benefit from improved improvisational, reading, and sight-singing skills. You might even start recognizing intervals from doorbells or car alarms!
Remember to relax, take your time, and have fun while learning!