Cantonese phonology resources?

Greetings,

I've recently started taking a conversational Cantonese class, and it's great fun. It's mostly going well, despite the fact that the teacher, an older gentleman from Canton City with fluent but not native competency in English, is a fairly traditional proscriptive grammarian rather than a linguist. This means, in practice, that he's great at drills and pointing out mistakes, but not as good at answering detailed phonology questions. And like a lot of native speaker language teachers, he doesn't seem to hear some subtle distinctions that seem pretty obvious to a linguistically astute foreigner.

I've noticed a great deal of contextual variation in both tone and vowel quality. To cite one example, tones that I believe are supposed to be mid-level often sound quite high to me in some contexts. Seems like it might also be emphatic.

Even more interesting, the vowel transcribed in the Yale system as o seems to vary between /ɔ/ and /o/, perhaps depending on tone, but I can't find the pattern described anywhere, and the teacher insists that the vowel remains constant, despite the fact that it doesn't sound like it to me. The only reference I've got, the 1970 Cantonese Basic Course from the FSI, describes this vowel as mid-back-rounded (/ɔ/) only, but it sure sounds like /o/ sometimes.

Anyone have any advice or suggestions for good, readily available, linguistics-oriented resources? I'm not a student, so things I can get in places other than university libraries are particularly appreciated.

Dō jeh in advance!