The best way to learn to read scores is to read scores. Start with simple scores and work your way “up” to the more complicated. And do not be surprised or discouraged if you find that some are much harder than others. I also have found that you read somewhat differently if you have “played” scores. If you are a pianist, play them on the piano; if you know solfeggio, sing them. These are useful supplements to simply reading them.

There are books on harmonic analysis, often focussing on particular works and particular composers, that can be helpful to a beginner. I do not recommend any of these in particular. Look in a good library and pick up those that interest you. Then, there are books that analyze the styles of composers and the architecture of particular scores. Analyzing the structure of scores helps you know them and remember them better. One book that I enjoyed is The Composer’s Advocate by Erich Leinsdorf. But other colleagues will recommend other books.