I was reared on Roger Sessions "Harmonic Practice" but Piston was more widely used at the time 40 or so years ago. I believe Hindemith's Traditional Harmony also had a wide following, but I might be wrong about that.

I cannot commend enough that harmonic study (especially if one is more of a keyboardist than singer) be complemented by study of sung counterpoint - classic species counterpoint to start. Why? Because instead of thinking "vertically" moment-to-moment the way studying fingers/feet might prefer, it forces you to consider sung lines, and what is more and less natural for amateur singers to do in the course of singing a line. There is so much music written for church in more recent times* that betrays a strong keyboard bias in composition and relative lack of engagement with the voice as the primary instrument. I see things on the page and it become immediately evident that a pianist or organist wrote this without a thought given to how ungraceful or awkward it is for a human voice to render it. Don't be one of those. End of rant.

* Oh, yes, I know there are examples from more distant times, but the worst examples weren't written for church but the theatre. Eg, a number of Handel's choral tenor lines in his oratorios. And yes JS Bach could slip into treating voices like string instruments. Et cet.

I cut my teeth on the three Walter Piston books ("Harmony," "Counterpoint," and "Orchestration"), and I lived across Alexander Street from Roger Sessions (in Henry Norris Russell's house) while a graduate student at Princeton. Personally, I am ever fascinated by Thomas Morley's "A Plain and Easy Introduction to Practical Musicke," which he dedicated to William Byrd, who had been his Master.