Making a bookcase look perfectly flush with the wall when the wall is not a true 90 degrees
One method is to take a board of the right height, plumb it with the bookcase next to the wall.  Then take a compass or something to hold your pencil at the right distance and scribe a line along the board following the wall contour.  Start at the bottom where it's closest to the bookcase.  You don't want to leave such a thin edge that it's hard to cut.  I'd probably want half an inch minimum.  Cut the scribed line with a jigsaw.  

Then attach to the side, flush with the front edge and it should sit against the wall with no gaps.  I might cut another board and put it along the side of the bookcase at the bottom so I can shove the bookcase into the corner and not stress the facade piece.