Ian Hiscock writes:
Theory is so important to being an independent musician. You don’t have to take your teacher’s word for it that a piece should be played in a particular way; instead, you can understand chords and harmony and how that affects the direction of the melody. You understand composition, phrasing and structure so that you get an insight into what a composer might have meant because you are also a composer yourself, working with the same issues.
Anyone working at grade 6 practical or above needs to have grade 5 theory, but there is no need to stop there, and anyone doing A level or more advanced performance should certainly study the higher theory grades. I was fascinated to do this, and I felt really enriched by the research I did into the set works and styles for my two theory diplomas.
The written exams are a good way of testing your own progress, but they are not necessary – some people are just curious about music and how it works, and theory lessons are a way to satisfy that curiosity.
musictechteacher.com – a music theory help website set up by Karen Garrett at Green Acres Middle School, Birmingham
musictheory.net – a series of free lessons and exercises, linked to paid apps
mymusictheory.com – a full set of free support material, organised by ABRSM grades up to grade 7. Also includes paid printable resources.
https://roselynsmusicstudio.com/theory.html – a music teacher in America with a very well-linked website – see this page for a range of theory resources.
Taylor, Eric: The AB Guide to Music Theory, Part I
Talyor, Eric: The AB Guide to Music Theory, Part II
Billings, Rachel: All-in-One to Grade 5 Music Theory Book (3rd edition) http://www.aaronpublications.co.uk/
Cole, Bruce: The Composer’s Handbook