The previous owner of our house was not a woodworker. Still, he left us some entirely adequate built-in bookcases in our family room. These were not fine woodworking, but still too good to tear out. (And considering that they installed hardwood floor after putting in the bookcases, yes around the bookcases, it would take a lot for us to go through the pain of ripping that all out!)
However, these bookcases were messy. We have a lot of books, we have a large family, and we homeschool. So all bookcases in our house get well used! But enough was enough. These bookcases were functional, but made the family room feel constantly messy. My wife put out the call for doors, and I was put to work.
For building doors for these basic bookcases, to turn them into cabinets, I elected to keep things quick and simple. The above photo is of the backside of my sample joint. The frame was made from 2-1/2" x 3/4" red oak stock. The ends were connected with pocket hole joinery. The inside was then cleared out with a 3/8" rabbeting bit on the router. A panel of 1/4" oak plywood is then fitted into this space and glued and screwed into place.
(I made sure I bought flatsawn plywood, and not rotary!)
More photos on my webpage