Someone on reddit brought this to my attention. I don't know why I never thought about this scene before, but in Mirai Fukuin there's a short scene where Mitsuru is alone in an empty room looking at a wheelchair that has the "The Vampire's Tears" book and a bouquet of flowers in it. After looking around the internet a bit more it seems that people theorize that Mitsuru's book is actually about his and Mana's lives using his precognition and that she passes away eventually.
Is there any "true" answer about what that wheelchair symbolizes?