I do not get the idea of buying a "book" to learn a *nix utility that explains itself in a single man page when all you need to learn is just a few keyboard shortcuts.
Obsessively focusing on the tool does not magically improve productivity.
Obsessively focusing on the tool does not magically improve productivity.
At least the manpages are openly reviewed.
The annoying thing about all these eBooks is that the effort would be better spent improving the manual pages for the existing product rather than yanking a few dollars out of some people for some "special treatment".