I came across this bookcase built into a doorway that looks really nice. The back side is obviously closed off as a flat panel that would need painting, although that could be trimmed out in various ways. The shelves are fixed and casing is used as trim with a pediment affect. Looks really nice. This comes from BuiltbyTodd.com.
Here is another one but is called a storage unit from Ikeahackers.net. This is well planned out and very proportional to the eye. The horizontal divider at head height adds a nice touch to the piece, along with the drawers with the horizontal pulls. Many homes have these spaces that are not utilized as such.
The combination of painted finish and exposed woodworking may have started as the following cabinets built around a window with a seat reveal. The addition of soffit over the cabinets to incorporate lighting and duckwork compliments the work. Noticeably the cabinets are a simple design which seems to match well in the room, from Customhomefinish.com.
Here is great example of combining a desk setting with bookcases on each side with a contrasting idea. The shelves being long are made thicker so they do not bow down over time. Again, incorporating the cornice into the existing molding around the room with a capital affect at each pilaster. The shelves look fixed and the dark blue adds a nice contrast for the piece, from Viewalongtheway.com. Also, using butt hinges likely ballpins or finials on each end with a unique type of pull that may be locking adds a bit of tradition to the piece.
Another example of an amazing bookcase tucked into a space with the wallpaper exposed behind the shelving to add a wonderful look to the piece. Being next to a doorway, you will notice the stop on the floor to prevent the door from banging to this beautiful craftmanship. Looking at the shelves, again one wonders if they are not also fixed, from Danbankscompany.com. The light fixture adds a nice touch, fluted pilasters, while the cornice trim is integrated into the existing bed molding at the ceiling edge. It may be that the wallpaper was installed after rather than before installation. This photo may have been taken before hardware if any was installed on the doors.
Bookcases and their designs are an integral part of any home, and they certainly inspire us to collect books and obviously read them, and this is in a world where book reading unfortunately has been declining.