If you're looking to dive into music theory, I highly recommend checking out some excellent online resources like Teoria, which offers comprehensive lessons, exercises, and reference materials. Musictheory.net is another great option, providing free online lessons and exercises that cover a wide range of music theory topics.
For books, "The Complete Musician" by Steven G. Laitz is a fantastic textbook that integrates theory, aural skills, and performance. "Tonal Harmony" by Stefan Kostka and Dorothy Payne is also widely used and provides a thorough understanding of traditional harmony. If you're looking for something more beginner-friendly, "Music Theory for Dummies" by Michael Pilhofer and Holly Day is a great introduction. Additionally, "A Creative Approach to Music Fundamentals" by William Duckworth offers an engaging way to learn the basics of music theory.