Unused electrical duplex box, soon to be hidden behind a bookcase?
Article 100 of the 2014 NEC is on point for you:


  Readily Accessible Capable of being reached quickly for operation,
  renewal or inspection without requiring those concerned to use a tool,
  to climb over, remove an obstacle or other.


But personally I'd move the outlet using an old-work box then cut a matching hole in the back of the bookcase.  If it's a normal stud wall construction you've got a 14 inch wide cavity to work with (subject to a wire length constraint).  One day you may appreciate having available electric power within the bookcase wall... lighted nick-naks anyone?

Or, move the outlet to the other side of the wall (per @TomG).

Abandon in place is possible, but beware that the wires might daisy chain to other outlets.  Any junction of any wires must itself be accessible by the above definition.  You can't really win :-).  If you want to abandon, places like the Berkeley Tool Library rent or loan circuit finders.  These come in two halves: one half plugs into the hot outlet, the other is a sensor that can trace the matching wire or breaker.