Not really useful
Theres really not much useful information in this little application; nothing that could not have been easily incorporated in a 10- or 12-page booklet. The buyer would be better off instead spending a few extra bucks to buy the excellent "Elements of Typographic Style" book.
Its not very well written, and has unfortunate spelling lapses such as "An em dash is one em wide, hence its (sic) name..."
slanker about
The Typography Manual