Phisherman writes:

Get a connector plate with 2 holes in it. Determine where (just above
the bookcase top) a stud is behind the wall and mark the wall and
bookcase. Move the bookcase away from the wall and screw the
connector plate into the top of the bookcase back such that half of
the connector plate extends above the bookcase top. Move the bookcase
back against the wall, lining up where you marked the stud location,
and drive a screw through he connector plate into the wall. Your
bookcase won't tip over (and is earthquake secure.) HTH

The last parenthetical point is not necessarily true. The
"authorities" recommend against rigid connection in earth-
quake country. Rigid connections are prone to breaking
whereas you want something that will "go with the flow".

And, BTW, everywhere is earthquake country. It's just
more likely in some areas than others.