To mix magic and investigation is something tricky because you have to convey to your audience what can and can't be done for them to know what to expect and try themselves to solve the case. The author just came out with everything out of his ass. He never stated that souls could be summoned, this would make the entire thing much easy, and for some reason, it wasn't done at the beginning. And he also comes with things like the location of the bodies, the letter, etc... It's a bunch of things that come far too easily because of magic. Such things make the entire case uninteresting because the case is solved practically without any deduction of the detective and he just uses magic to solve the case.
This thing was already a shitty copy of Shield Hero anyway, but even when it tries to be original it does in a shitty way, I'm dropping it.