As long as the comment is useful and contributes, I think it's fair game. This definitely qualifies. That said, if it's not free, I personally would prefer that be mentioned, (and at the same time a link to a free sample chapter or equivalent if it exists would be useful).

90% of HackerNews is self plug ... so don't worry :)

It was quite a good book! It might be due for an update, what with tmux plugins and other new tools. I'd buy an updated version.

Thanks! It was most helpfull when I got to know tmux.