
Tonal Harmony, written by Stefan Kostka and Dorothy Payne, is a key text in musical theory that clearly and structured the concepts of tonal harmony, from the basics to more complex ideas. It includes practical exercises, analysis of scores and a historical approach that enriches learning, making it a resource widely used in music and universities schools. In addition, it usually offers complementary material, such as study guides and online resources, which reinforce the concepts treated.
If you want a decent, comprehensive dive into music theory and composition you can pick up any old edition of Tonal Harmony.
I recommend sitting in on a university music theory course if it all possible. But, if not, I can at least recommend the textbook we used: Kostka and Payne's Tonal Harmony. If you want to learn slowly, from the beginning, it's a great resource.
Our instructor told us that, by the end of the first semester, we would be writing four-part chorales and that we'd sound like Bach. That sounded thoroughly ridiculous and I thought he was full of shit. But, no, he was right -- after a semester I was able to do precisely this.