How do I build shelves on non-square (>90 degrees) walls?
"Not square" can mean two different things: not a right-angle, or rounded.

If the walls are not at right angles, you can use a T-bevel to measure the angle between the walls.  If you don't have a T-bevel, you can hold a piece of light cardboard against one wall and fold it so the crease is against the other, which you can use as a template for cutting the shelves.

If the corners are rounded, I'd use the cardboard again: estimate the radius of the corner, cut a quarter-circle of that radius, and check how well it fits into the corner.  If necessary, adjust the estimate and repeat.  Then use the quarter-circle as the template.