Bill Tindall, E.Tn.
The joints are largely end grain. An end grain joint can be moderately strong if it is not glue starved. Precoat the end grain, let set a bit to allow all the glue that can penetrate into the end grain, add more glue if necessary and clamp. ("necessary"...either all the glue has penetrated into the end grain or the glue has begun to dry on the surface. I add more glue just in case. )

A spline will strengthen the joint but only come into play if the case was dropped. Or could be added as a decorative element, not something I would do.

A project worthy of postponing the book case for a bit.