Mainstream books carry good information on all the characters under discussion here. In fact, it's clear that the parallels are real and valid.
For example, I learned about the Jesus-Mithra connection from my Classical Civilization professor at the university I attended a good while back. I then went to the library and found some sources verifying it, primary sources even. If I went to the library now, I could do the same thing.
I'm sure any history of Persian or Hindu belief systems would tell us everything we needed to know.