Marli
You should ask this question in Religion and Spirituality if you want an answer based on the doctrines of the Latter Day Saints.
I'm not an LDS (aka a Mormon). I don't know who Porter Rockwell was, if he was great in his church or an apostate, if John Rockwell wrote about him as a fan, a loving son, or a person with a grudge against Porter or the LDS.
Was the book well written? Would it be useful to a curious non-Mormon or someone curious about John Porter?