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I'm a teacher, and so far it's been extremely easy teaching beginning music theory to all my new students, but I want to know more music theory, especially when my students get more advanced. I've learned through nancy faber books my whole life, but afterwards my teacher just had me play solo pieces and that was it. No more learning music theory. Is there any books for advanced pianists like myself where I can learn advanced/complex music theory? Sorry if my question does not make much sense
I like Robert Greenberg's various Teaching Company courses.

If you're not too advanced, you might get value from his Fundamentals of Music course (which sometimes goes on sale). If so, be sure to download the PDF course guide as well.
https://www.thegreatcourses.com/courses/understanding-the-fundamentals-of-music
My teacher used the ABRSM music theory work books with me, which took me to level 6 at which point I stopped. The books have sections for pupils to complete (home)work, and there are also teachers answer books. At the end of each book you can take an exam paper formally, or informally and for the latter past papers are also available with answers. I used to do the work books at home bit by bit with teacher marking at start of lesson, and the exam as an at home open book style but trying not to actually use the books. To complement the music theory work books there are two compact size and quite terse reference handbooks.

Edit to add, these are generic music theory not aimed specifically at piano, so cover things like tuning and notation for orchestral instruments, lyrics, composition, transposition, etc.
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