Anyone looking to do Music at Uni, I was told that one of the absolute key books that you must read is 'The Classical Era' by Rosen. Tovey's essays in musical analysis are fairly light too, quite opinionated so be careful, but they're usually 5 or so pages on a particular work, just giving you a brief outline, so it's a great way to gain a general knowledge of major works. Listen to them before you read the corresponding passage though or it makes no sense of course.
Personally, I tried to pick a few areas I'm interested in (Soviet-era music and modern interpretations of it, the social context of Mozart's operas, orchestration and the development of harmony) and then looked around for the standard textbooks on each of these. I'm working through Lovelock for harmony just because the A level syllabus is so damn boring, reading a biography of Shosi which talks a lot about Politics, a book by Henze called 'Music and Politics', one by Dent called 'Mozart's operas' and two textbooks on orchestration, just noting one by Carse at the mo. Also skipping through the Stravinsky conversation books.
Have you acually tried approaching teachers about extra reading? Mine didn't say anything to me until I asked them myself. Possibly they assume a lack of interest unless you express it, as so few people care about Music at a high level. Before I asked my teachers I was getting books from my parents, I'm lucky as they're both music teachers.