I'm going to take a crack at building the shelf pictured below, except I'm just going to make the top section, since I only want one level.
I was thinking of using a french cleat behind the length of the taller board to mount the shelf to the wall, but I realized that will hang the entire weight carried by the shelf off of the joints where that board attaches to the end frames (yellow highlight).
My plan was to use dowels at all the joints, but I'm not sure how much weight they can carry. If that board is 1" x 2", how many of what size dowel should I use?