Aaron Rabushka wrote:

>Perhaps it was something else (other than the FitzWilliam) that he (Bach)
>copied by moonlight.  I'd heard that he copied a book that he had to sneak
>from his brother who wouldn't let him use it.  It's been a long time since
>I've heard that story, and I don't remember the source.

Says The New Grove:

   "....Several biographers have told the story of how [Johann Sebastian's
   older brother] Christoph would not allow his brother to use a certain
   manuscript; how Sebastian copied it by moonlight; how Christoph took
   the copy away from him; and how he did not recover it until Christoph
   died.....Later authors, knowing that Christoph lived on until 1721,
   and that the brothers had been on good terms, have tended to reject
   the story--perhaps unnecessarily, for it may illustrate contemporary
   attitudes to discipline and restraint.  In fact, the story fits in
   well with the little that is known of the Ohrdruf years, and with
   the idea that Sebastian taught himself composition by copying.  Most
   probably he recovered his copy when he went to Lueneburg.  As for
   its contents, Forkel implied that it contained works by seven named
   composers, three of them northerners.  He probably misunderstood
   Emmanuel's reply to another of his questions; according to the
   obituary, the manuscript was exclusively southern (Froberger, Kerll,
   Pachelbel)--as one would expect since Johann Christoph had been a
   Pachelbel pupil...."

Walter Meyer