I wanted one clean shelf with no visible brackets. The answer is a French cleat hidden inside the shelf body — it carries the load and disappears into the wall.
materials
Wood
Black walnut
Joinery
Hidden French cleat
Finish
Hardwax oil
Build time
One weekend
verdictHolds a full row of books without a sag and looks like it grew out of the wall. The whole trick is a 45° rip and a level pencil line.