The piano book is fairly specifically targetted at issues that a
pianist would care about. Chord voicings are the topic of a number
of chapters. I suppose anyone would learn something from it, but
if you're not a pianist I wouldn't spend the bucks on the first.
That being said, if you ARE a pianist I think its worth having
both books, there are just not that many comprehensive books
on theory out there. But the larger Theory book would be my
choice if it had to be one or the other. There IS some overlap,
but each has enough material that is not in the other book to
make it worthwhile.
Cheers,
Jack