[homebody]

I'm wanting to use the Billy bookcases as a half-wall with the open side facing our hallway and the back facing the living room and woodstove. Should I have it installed and then drywall the back or finish it with plywood or...? How can I tie that together to finish the top? Would there be another option for the half-wall that would work better?

[splatgirl]

Basically, what you're describing is a built in bookcase/halfwall.
Applying drywall directly to the back of a Billy carcass won't work.
Depending on the level of permanence you want, you could either frame out and drywall or build out using 3/4" finish plywood, a custom sized opening that the Billy will slip into (most permanent) or just plunk it where you want it and then apply something like plywood directly to the surface of the top , back and side of the carcass (least permanent). Aesthetically, the back is the only thing you'd really NEED to cover up, and that could be as simple as carefully cutting a piece of 1/8" masonite to fit the back, gluing it on and painting it.