I do still have a copy of The Devil's Avenger. Put it on the market before, for whatever reason I haven't sold it. I would be willing to consider offers if someone really wants the book and will appreciate it. Send me a private message if you want to.
The strengths of Secret Life of a Satanist
are immediately apparent. It was written later and therefore covers more ground on not only Anton LaVey but the history of the Church of Satan
in general. The writing is solid, it's fun, it's just all around a good read.
I would say the way in which The Devil's Avenger stands out is in the literary descriptions of Anton LaVey and the rituals he constructed. Rituals that aren't, as far as I'm aware, in any other literature to such a degree. Very fun rituals, ones that aim to infuriate some and make others laugh.
So I found myself more educated by "The Secret Life of a Satanist" and more entertained by "The Devil's Avenger." Both have strengths, neither are weak.