I'd highly recommend the "singing against a drone" exercises in "the harmonic experience" by W.A. Mathieu. You can do them with an acoustic guitar, and they wil

I'd highly recommend the "singing against a drone" exercises in "the harmonic experience" by W.A. Mathieu. You can do them with an acoustic guitar, and they will improve your ear immensely. The first exercise is simple: singing exactly in tune with an open string.