Personally, I don't really blame you or think less of you for this screwup. I never got the impression that you are the sort of person who should be sent confidential book review drafts. Maybe you'd disagree, but that seems like a misunderstanding of your role to me.
It seemed clear to me that you made yourself available to confidential reports regarding conflict, abuse, and community health. Not disagreements with a published book. It makes sense that you didn't have a habit of mentally flagging those emails as confidential.
Regardless, I trust that you've been more careful since then, and I appreciate how clearly you own up to this mistake.
I want to offer my +1 that I strongly believe Julia's trustworthy for reports regarding Leverage.