A teacher is by far the best way to go, failing that The Complete Musician is the best book/source I have seen for self study. The problem with self study is that the basics of harmony/triads seems very simple and people plow through it and think they understand than get completely stuck. Everything is built on harmony and if you do not understand it you will not progress. If you go with self study remember that if you are stuck it means you did not understand what came before, go back and figure out what you missed, which is where a teacher is very handy since they will have a much better idea about what you missed than you do.