This was Rushdie’s first novel and it’s very much a journeyman work. He does use the magical realism vehicle that he uses much better effect in later novels (such as the excellent Midnight’s Children and The Satanic Verses). It’s difficult to identify themes that that he would come back to so it’s best to just look at this as it comes.
Flapping Eagle takes the elixir of immortal life, after his sister is first given it and spends the next several hundred years wandering the world, after her disappearance. Eventually he finds his way to Calf Island, which exists somewhere between dimensions and eventually to a confrontation with the eponymous Grimus.
The characters that Flapping Eagle encounters on his journey are mostly just caricatures, without much in the way of depth, although Flapping Eagle’s companion and guide Virgil Jones does get more development.
I think that Rushdie may have been going for somewhere between gothic and grand guignol in this novel, and to some degree he’s managed it, but at the expense of any warmth or engaging characters. Flapping Eagle is a difficult character to warm to, as his motivations and thoughts mostly go unreported, and his actions are often less than endearing. Rushdie’s writing here is workmanlike but he’s still developing a craft. It’s not yet the polished and poetic style that it would develop into.
So mostly worth reading if you’re a fan of Rushdie to see how his writing developed, but it doesn’t really stand up to his later work either in plot or in the writing.