give good answers. In truth (and Abrahamists hate
to admit this), the intellectual foundations of Christian theology are completely built on the works of Plato, Aristotle, Plotinus, etc. and other philosophers of this class. If this was stripped away Christianity would certainly be very, very intellectual impoverished. So to start with the very foundations of philosophy itself is a very good suggestion. In doing this, you will quickly see that certain ideas are not Christian at all, but far predate Christianity / Judaism.
The Bhagavad Gita is another essential text, as the other anon says. If you are only to read a couple of things from the East, this should be one of them. Personally I have read it three or four times now, and with each rereading I appreciate it more and more, and actually grasp it a bit better, so it is very good. I believe that being familiar with at least some of the Upanishads is worth it too. Sometimes they are extremely difficult to make sense of what is being said, other times they are quite lucid. These too benefit from re-reading.
I have yet to read the Avesta unfortunately. Do you happen to know a good translation or anything?