I'm glad that after my Rock/Metal early days a teacher recommended me the Adler, because it turned out to be the best book for me. I took that, tried to round up as many examples as I could for audio, and combined it with an intensive study of first Beethoven's early string quartets, then progressing up to the 9th Symphony and other heavy duty stuff like Wagner, Mahler, Schoenberg, Bartok, and Bruckner. Starting with (relatively) simple string quartets are a terrific way to get down your four-voice composition chops...in fact, there's a fantastically helpful chapter in the Adler about Haydn's incredible "Emperor" quartet.
I ended up studying my way out of guitar and Rock lol! For the early part of my life guitar and that music were my greatest loves...after all my studies I saw behind Pop music and it's tiresome fashion-consciousness and limitations in form and fell in love hard with the orchestra and Art music. The book opened up this glorious, capricious, infinitely wide and deep vista for me, and I'm grateful it came around in my life when it did. I cared more about making great music rather than a great Metal and/or guitar song. I came to see how guitar is just one instrument, and there are others which are easily as wonderful and more than worth study.
I must admit though, Rock can be a gateway into better music, and I'm glad guys like Blackmore, Rhoads, Uli Roth, and Malmsteen brought their quasi-Baroque thing to the classic Metal/Rock/whaddevah-ya-wanna-call-it table (btw folks, no matter the designation, it's ALL Pop structurally, besides the deliberately cantankerous fusion and "Prog" genres).
So, Adler killed Rock n Roll for me lol! Just kidding, I still break out my beloved oldies every now and then.