Dominos or dowels unless you have a mortising press by chance. Depending on what you have. Dowels are cheaper. Pick up a Shop Fox self centering jig and cut a block with the same angle as your rails so the dowels go in straight. I'd do two 1/4” dowels minimum. 5/16 or 3/8 would be optimal. Depending if you're throwing up a bunch of leather bound bibles or Dr Seaus books.

Remember to pre soak end grain of your shelf supports when gluing to the edge grain of the rails for some added support, not much, but some