Boring. The Tmux spec is short and easy enough to read on one's own. Thus what I wanted was a compelling reason to use Tmux, to be sold. I didn't get that. Aside from remote sessions(?), it seems Vim/Emacs can do everything Tmux can do, from window management, terminal emulation, scripting language, etc. And to add insult to injury, Vim/Emacs don't require learning a new DSL. Huh...
I'm either 1) right about Tmux being shit, or 2) wrong about Tmux being shit. Notice, however, that both imply that this book is shit ;)