Music theory

The theory course from the ABRSM is a very good start. 

Narcis Bonet, Nadia Boulanger's student, wrote two deep and pithy books on the subject: The Essential Elements of Music and The Fundamental Principles of Harmony (translated from the French).

There are schools of thought on solfège; my teacher has me using fixed do, which I've come to deeply appreciate the farther I go in my studies, and which is standard in most of the world, though not in this country (outside of Julliard and Mannes School of Music and a few other places).

Anne Crothers Hall has an amazingly rich book, Studying Rhythm, that will set you up on that side of things; best to work on this with a teacher, in my experience.

Another wonderful resource is Evan Fein's Inner Hearing: the Systematic and Progressive Approach to Sight Singing (which is agnostic with respect to which solfège system you use).

I also love George Dandelot's Manuel Practique Pour les Études des Clés, but I don't think it's been translated into English.

Two more books that will help you read scores and C-clefs are Melcher and Warch's Music for Score Reading and Morris and Ferguson's Preparatory Exercises in Score Reading. My teacher has me sing a line on solfège while playing another line on the piano, advancing when needed to playing two and then three or more lines while singing a fourth.