OK! When I browse through your code a second time,
I realize that you have already pointed out the problematic
line yourself:
\score {
\context PianoStaff <<
\pageBreak % Has to be here, but bug seems to create extra
empty staff
First of all, the \pageBreak command doesn't work at the top
of a score. Please read at
http://lilypond.org/doc/v2.7/Documentation/user/lilypond/Page-breaking.html#Page-breaking
to learn what to do instead.
Also, the \pageBreak command, as many other commands,
needs a Staff context to "live in", therefore LilyPond creates
one, which explains why you get an extra stave.
Third, if you only have a single \book{...} in your file, then you
can just as well remove it. LilyPond will silently add a \book{...}
around each .ly file if there isn't one. The only reason to explicitly
use \book{...} is if you want several output PDF files from one and
the same .ly file, since if you have several \book{...} blocks, then
you get one output file for each such block.
/Mats