He would have been fine with...a phone. "Talking about my book." Or, maybe, IM with OTR without logging, but that might arouse suspicion if non-default, and might also fall prey to crazy counterparty either not disabling logging or intentionally recording and blackmailing.
I thought this was one of the benefits of OTR over, say, PGP. That is, with PGP if you sign a message the counterparty can wave it around and say "look at what this poerson said", while with OTR, since the encryption is done with a shared key, it could be just as likely that the counterparty made it up.
At least, that's what I gleaned from this wonderful video someone linked to on HN yesterday.
So, I mean "use OTR, and ALSO disable client logging".