Arnold Schoenberg's Fundamentals of Musical Composition is excellent, definitely on my short list of favorite music textbooks.

If you liked that and found it valuable, you may also like and value these:

- Paul Hindemith's The Craft Of Musical Composition
- Joseph N. Straus' Introduction To Post-Tonal Theory
- John Rahn's Basic Atonal Theory
- Charles Wuorinen's Simple Composition
- Robert Morris' Composition With Pitch Classes
- Reginald Smith Brindle's Serial Composition

I feel like my list would be remiss if it didn't also include
- Allen Forte's The Structure Of Atonal Music, and
- George Perle's Serial Composition And Atonality
...but tbh, those two books are way more math-intensive than any musical tome I personally would consider "essential" ...they are awesome reference books, and they are considered the definitive treatises on important 20th Century compositional techniques, but they are deep and heady and to my mind not quite the sort of text that inspires or reveals or provides the curious composer with the "ah-ha!" moment that Schoenberg et al's books do.