Reddit mentions of Guide to the Practical Study of Harmony (Dover Books on Music)

Peter Ilyitch Tchaikovsky, Guide to the Practical Study of Harmony

Arnold Schoenberg, Fundamentals of Musical Composition

Music Composition For Dummies

The Complete Idiot's Guide to Music Composition

Music Composition 1: Learn how to compose well-written rhythms and melodies (Volume 1)

More at Amazon.com

https://www.reddit.com/r/Composition/

the reviews aren't really wrong... it does have its flaws, but there isn't really another book that does it better. i've read quite a few harmony books and it is the most comprehensive basic harmony book that i've found. it's also pretty much the standard for college courses.

Tchaikovsky also wrote a book on harmony. It is good and very inexpensive, but very short. he writes extremely efficiently though, so there is really a lot of info in such a small book, but obviously not as much as Tonal Harmony. It's also older than Tonal Harmony, so some more modern ideas are not included. That said, it's a great way to quickly learn a lot, and at the price it's really a no-brainer. It doesn't have exercises or lots of examples... just good info.

Also, I should mention that all these harmony books teach using the classical tradition of placing heavy emphasis on voice leading. If you are, for example a guitarist writing pop and rock songs, you may not see how the information will be relevant to what you do, but I would encourage you to go ahead and read through it, as it will make your writing better, and give you a more complete understanding of music. Also, there isn't really a good basic harmony book that doesn't teach this way.

Also, if you are interested in Jazz Harmony there is absolutely no better book than this one.