I recently read an interview with Jazz pianist and educator Hal Galper, and found that his views on practicing really mirrors my experience as well. Here's what he said:
It's so easy to get stuck in following a set path when learning how to compose since it's so much ground to cover. While on the surface it might look like a great idea to separate each subject and "master" them before moving on to the next (I don't believe we actually master anything, we're only on a different stage along the path), it's quite far removed from how you're actually going to use the skills you develop in your day to day composing, arranging and orchestrating.
I think that's part of the reason why so many of us have a harder time composing after a period of intense study than we had before it. This can lead to the conclusion that you lose your creativity by "learning too much", which of course is totally the opposite.
What has helped me to avoid this problem is to focus on the music that I love regardless if I feel I'm "ready" for it or not. Whenever I'm drawn to a particular piece of music I do whatever it takes to learn it. I don't care if the harmony or counterpoint is way over my head, I just transcribe it and analyze it in any way that I can.
This fuels me to spend the time improving whatever area that particular section of music exposed as not totally clear to me theoretically. Here's where all the different theory books really helps and where it's a good thing that they're separated by topic and organized in terms of difficulty. I use them more like an encyclopedia than a "take me from a to z in all subjects in a strict linear fashion" type of approach.
Doing it this way forces you to consider all aspects of composing at the same time and you start seeing in a more practical way how each aspect of it informs the others. While it might look pedagogical on the surface to separate each subject, it's nowhere near the way you'll actually use the things you've learned when staring at the blank page.
Another thing I've found is that the reason that I'm drawn to a particular piece of music, is the fact there's some aspect of it that I don't understand yet by just listening casually. So most of my learning nowadays (probably all of it) is motivated by a curiosity rather than following a measured menu of different subjects compartmentalized in big blocks.
Incidentally this is also supported by recent studies in learning, the term for it is Interleaved Practice and you can read more about it here.
The tricky thing is that by not digging down and practicing a specific subject in a linear fashion it feels like you're not progressing as quickly. As you can see in the article above that's not the case. They've found in several different studies, that you are much better off in the long run by mixing up your practicing/learning. Even though the day to day improvements aren't as clear as when compared to the block type studying, you actually learn faster and in a deeper way by mixing it up once the information has had time to settle.
All this being said I still think that having the information organized in a linear fashion and separated by topic is a very valuable resource to have. I just don't think it's a practical nor fun approach learning it by rote in the way it's laid out. It's like trying to become a good writer by memorizing an encyclopedia and studying grammatical rules, without ever reading something that you actually enjoy and aspire to write yourself someday.
If you're anything like me you'll find this very freeing and you'll finally get over the overwhelming sense of "there's too much to learn, where do I start?!".
Start with the music you love and remember that you'll learn faster and better by having fun and being engaged in the topic at hand. What a concept:)