This might sound somewhat strange, but I used to listen to distracting music (e.g. rock) when practicing maths. I had no idea why that worked for me, but on hindsight, I think the idea is that if you can train yourself to perform under divided attention, you should have no trouble doing it in slightly stressful situations (like an exam). Rock music also seems strangely motivating for me.
I listen to non-vocal music (classical music works best) when I have to read deep texts (philosophy-related stuff, mainly) where you have to hold a whole mess of ideas in you head so what you are reading makes sense. Otherwise I just play my mp3 collection (mostly rock+indie) in the background for normal study, which for me is bio-related stuff and empirical papers, which tends to be more bite-sized and require less engaged processing. It also stops me from sub-vocalizing everything with the result that I end up reading faster.
Probably, the best ambient study environment would depend the cognitive load imposed by the study material, your own attentional resources, and your mood.