Step 1: Materials
The cabinets are pre-built and 36" wide and about 13" deep.
Melamine laminated sides - they are sold as 12" wide, but are actually 11.75" wide. These are pre-drilled with shelf pin holes.
Plywood top & bottom & shelf above cabinet - we used 3/4" hardwood laminated - Luan or Birch is fine.
Plinth blocks (5" high and 2.5" wide) and rosettes (2.5" square)
Fluted molding - 2.25" wide
front edge trim molding
bottom and top trim 1"x3"
shelves - 3/4" plywood with 1"x2" front edges
shelf pins and knobs for the cabinet doors
Step 2: Section Frames
The cabinet frames are pretty easy to make. We didn't use any fancy joinery, but you are welcome to! :-) The cabinets have three fixed shelves; one for the bottom of the cabinet, one for the top of the cabinet, and one at the top of the frame. The frames go from the floor to almost the ceiling, and the cabinets fit inside the frame.
The length of the fixed shelves is the same as the width of the cabinets - in our case, 36". The width of the top and bottom fixed shelves is the same as the melamine sides (11.75"). The width of the middle shelf on top of the cabinets is wider to get some overhang over the cabinets (noted below).
The top edge of the bottom shelf was set to the height of the baseboard + 1/4" for a reveal between the baseboard and the bottom shelf. Since we used 1x3 boards on the bottom, that's 2.5" + 1/4".
The top shelf height is set to allow room for a top face molding, and then crown molding to the ceiling above that. The height of each frame was a bit less than the ceiling height to allow for moving the frames in. For our shelves, the top edge of the top shelf was the ceiling height minus the crown molding height minus about an inch reveal to the shelf minus a 1/4" reveal from the face molding to the top shelf.
The shelf above the cabinets needs to be deeper than the melamine so it overhangs the cabinets a bit. We made it 13.5" deep. It's wider than the melamine (11.75" deep) to allow for the plinth block (3/4") + a 1" overhang over the cabinet fronts.
At the top back of the sides, we cutout a notch to allow the existing crown molding to stay in place. This seemed easier than removing the molding, and since we have an older house, that crown molding is probably nicer than current materials. Maybe some future owner will tear out these bookcases and thank us for leaving the molding :-)
We used drywall screws to hold the sides to the shelves, and they definitely need to be pre-drilled and counter-sunk since it's going into the edge of plywood and we don't want it to split.
Each frame is not very strong, but once the cabinets are in place and they are side-by side, the whole system gets a lot stronger. Attach the top and bottom shelves, but leave the cabinet top off for now - that can be added on top of the cabinets.
Step 3: Cabinets
Placing all the frames against the wall, and use shims to get the cabinet top shelves to line up and look straight. Once you have them lined up, use a few 1 1/4" drywall screws to connect the frames together on the sides so they don't shift. Pre-drill and counter-sink these holes too. We found white plugs to put in the counter-sunk holes. Wood putty and paint would probably work too - might not match the melamine finish exactly, but you really wouldn't notice once the shelves are in and filled with books.
Step 4: Details
The baseboard is 1"x3" pine, and the top face molding is too. Each is attached with a 1/4" reveal from the bottom and top shelves. The baseboard does not need to touch the floor (but it can) since we add shoe molding on the face.
We added crown molding to match the existing room molding - some tricky cuts where they meet, but we used the coping method to match that.
On the front edge of the shelf above the cabinets, we added some trim molding - it's 3/4" thick, so matches the front of the shelf.