Music theory, terminology, etc.
Is there a good book or two out there that can completely explain musical concepts and terminology? I always hear words like tremolo, arpeggio, pitch, mode, note, step, key, etc and get curious. I’m a beginner DJ/producer and I’d like to know more. 
Book or two? No. Books? Yes.
As for what? Pretty much any book to do with music will at least fill you in on most of it. It’s not just a dj thing, it’s a musician thing.
Get something to do with piano or guitar, maybe something about drums too wouldn’t hurt.
Grove’s Dictionary, yo.
Hit up the music section of your local bookstore, or go to a music shop (long and mcquade, guitar center etc.) And see what they have. Or read up on music theory online.