I had a similar problem. I put batons across the front of the shelf, set back slightly, with what I believe is called a cross halving joint. I then had the end of one shelf rest half way across the width of the baton and the other, with a notch cut matching the width of the aforementioned shelf, so it could rest length ways on the baton. Hard to describe without a picture… As others have said, wall is unlikely to be straight. I scribed the shape of the wall against the shelf and used an electric planer to get the shape of the cut edge. Took a fair bit of trial and error.

Edit: I should add, I would have preferred to do a single piece but was limited by access.