alexy wrote:
I have a buildt-in bookcase with a 1/2" ply back that [nearly] meets
up with a wall that is slanted in at the top at 45 degrees. The back
of the case and the wall are painted with the same flat wall paint. I
want to put a cove at the junction of the plywood and drywall,
probably with a 2-3" radius. That means that the material I build the
cove out of will be up to 3/8" thick. I have both plaster of paris and
drywall joint compound. Which should I use and why?

Use both. Do the first coat with the plaster of paris (which is
basically what setting type joint compound is)over fiberglass tape and
then use the premixed joint compound for the finish coat(s).

Why? Plaster will shrink less and dry harder. Joint compound shrinks
more, but sands easier.