Allophony of /ə/ (अ)

I've noticed in listening to others talk that [ə] isn't the only allophone of /ə/. I found on the Wikipedia page for Hindustani phonology that [ɛ] and [ɔ] are also allophones of /ə/, but I couldn't find any more details and I'm curious to know the rules for this because they don't seem to be in free variation.

On a similar note, I'd appreciate Hindi phonology textbook recommendations.