Seraphita has Swedenborg at the root of it- and teaches reincarnation, the vanity of human wishes and that the soul's true end is to be reunited with God. A typically Balzacian, pushy young man (unfortunately called Wilfred) is confronted and counfounded by the eponymous Seraphita- an angel in androgynous human form. Words and words and yet more words are poured out in an attempt to say the unsayable.
This work exists in the same universe as Splendeurs et Miseres des Courtisanes?
Yes it does. And this is why Balzac has to be read in depth. There is no single book that contains the whole of him. Every text needs to be read in the light of every other text. His masterpiece isn't any one book but the entirety of la Comedie Humaine.