How to fill gap caused by walls not being 90 degrees? ❓

My wall is not perfectly square, and I do not want to tilt the shelf backwards. The shelves are currently level, the wall is not. How can I fill in the little gap to the wall?

Once the project is finished, I intend for these to look built in. Will be adding baseboards and trim to the top. Just not sure how to fix this issue with the gap.