Low and long bookcase, or tall and narrow bookshelf
A bookshelf is the only furniture that gets more interesting as it fills. Everything else looks best empty. This one is a frame waiting for content, and the choice between a low horizontal bookcase and a tall vertical one decides what the room does long before a single book goes on it.
Low and long draws the eye sideways and keeps the ceiling feeling high. Tall and narrow does the opposite. It pulls attention up and makes a small room read taller, at the cost of some visual weight against the wall. Neither is right in general. The same proportional thinking runs through the mid-century edit, which has always preferred the low horizontal where it could get away with it.
Choosing the proportion of a mid century modern bookshelf
The low bookcase is a different piece of furniture
A three shelf bookcase at around sixty inches wide stops being storage and starts being a surface. The top becomes a place for a lamp, a record player, art leaned against the wall. That is a genuinely useful thing in a room that has run out of horizontal space, and it behaves much more like a mid century modern credenza than like a bookcase.
The mistake is buying tall because the book count says tall. Count the books you will actually keep on display rather than the ones in boxes. Most people need half the shelving they think they do, and the half they skip is the half that would have made the room feel crowded.
Open back bookshelves, or against the wall
An open-backed bookcase lets wall colour show through and keeps the piece visually light, which is why it works as a divider between two zones. A closed back reads as solid, holds the eye, and is the better answer when the wall behind it is doing nothing for you.
What goes on mid century modern bookshelves
Leave a third of the shelf empty
Fill about two thirds of the run and leave the rest open. Books packed edge to edge on every shelf read as a wall of spines, and the bookcase disappears into its own contents. The empty third is what lets the frame stay visible, and the frame is what you paid for.
Mix horizontal stacks with vertical runs. A few books laid flat break the rhythm and give you somewhere to set an object. This is the nuance most shelving advice skips, and it is the difference between a shelf that looks styled and one that looks lived in.
Wood tone, and the desk the bookcase sits near
Walnut, oak, mango and bamboo each throw a different warmth under lamplight, and bamboo in particular reads lighter and more uniform than the hardwoods. In a mid-century modern office the shelving usually sits within sight of the desk, so keep the two in the same tonal family. A walnut bookshelf behind an oak mid century modern desk is a near miss you will notice every working day.
Arched, asymmetric, leaning bookcases
An arched top softens a hard-edged room and does the work a piece of art would otherwise do. Asymmetric shelving breaks the grid and gives the eye somewhere to stop. A leaning shelf asks almost nothing of the wall and suits a rental or a room still being figured out.
All three are decisions about attention. A plain rectilinear bookcase recedes. Anything with a gesture in it steps forward, and a room can only carry one or two pieces that step forward before it starts to argue with itself.