1. why does a screw sometimes keep on turning?

Because the hole is too big (either the drill wobbled, or you are in soft
mortar that fell out). If the problem is slight, just shove wrap some toilet
paper round the rawl plug, or shove some PVA down there to glue the rawlplug
in place so it doesn't spin.

Another possibility if you don't want to have to remove your bookcase
from the wall to get at the rawlplug is to use a different type of
screw. A lot of modern screws (particularly those billed as chipboard
screws or recommended for use closer to the edge) have a thinner shank
than the standard traditional woodscrew and I find these don't work so
well in rawlplugs unless the hole for the rawlplug is drilled slightly
undersize.

Martin