Glue and screw would probably be enough IMHO. The concern with screwing into the grain isn't the shear force (i.e. the weight of the shelf pressing down perpendicular to the screws), it is the pull force parallel to the screws. You won't get much pull force parallel to the screws, since the screws along the other edge/across the grain will resist that force.
Just don't overtighten.
Given you've got a few shelves and it would be worth the investment in making a jig/guide, cam fasteners could be another option. But they're ugly if you can see them.