I don't know if you have access to University Lecturers... if you do, go have a chat with
the software engineer lecturer. I've only had positive experiences when approaching them
about "totally-unrelated-to-course" projects.

On the other hand, I bought a book (forget the exact name.. can find out)
which showed some of the basic ObjectOrientated stuff, but at the same time,
I found it to be relatively useless when trying to apply it to "music-software".
(Ie: Ardour, Seq24, Dino, etc kind of programs)

Spending time doing out program diagrams.. (you know the "standard" boxes approach
to explaing how classes interact.) That's been my approach, I didnt really find any great
resources online. If you do find any, please post back here! :-)

Good luck, -Harry