The IVAR system from IKEA offers shelves and pentagonal corner units, but I wanted a little triangular end unit. With a bit of sawing and drilling, I was able to transform each small shelf into two triangular shelves.
As an aside, note that the zigzag long shelves add some extra structural support as they act partly as a truss, in addition to looking interesting.
Step 1: Start with small shelf
IVAR offers a small pine shelf which has reinforcing metal bits for supporting the shelf on little metal pegs.