Books by historically significant composers on composing are few and those that do exist are uneven in their approach. My favorite, probably, is Olivier Messiaen‘s book, Technique of my Musical Language. You probably won’t find it for sale anywhere and if you do, it will be prohibitively expensive. But, you do occasionally find it in major libraries. Also, most university libraries can get it for you on interlibrary loan from somewhere like the Eastman School of Music Sibley Library (I notice there’s a link on that page to interlibrary loans). Note, the copy at the Sibley Library (Eastman is where I got my master’s degree) is the traditional two-volume version (text in one volume, examples in another) — be sure to get both volumes.
What I like so much about Messiaen’s approach is how thoroughly practical it is. He doesn’t justify anything. He just describes what he does. It’s quite literally about how he composes. He only describes some of the things he, himself, does, certainly not everyone. It’s the most honest book on composition I know. It is completely without pretense.
If you get a chance to look at this book, I recommend it to you. It may give you some ideas.
Here’s a YouTube video with score of Book Two of his Catalogue d’oiseaux.
You might discover you don’t like his music, but you do like some of the ideas in his book. That’s okay. That can happen! Don’t let prejudice against his style blind you to what valuable ideas you might discover in his book. And, you never know…someday you might find you like, at least, some of his music. I know I do.