Mantissa is a short, light romp through the writer's mind. There's no heavy subtext to mull over. There's no ponderous character development to follow. There's just Miles Green in his hospital room, which becomes other things, and Erato, the woman who is his muse. A few other characters lurk in orbit around the room, but the whole story takes place literally in the brain of Miles.
Most of the book is dialogue between Miles and Erato as he alternately romances and berates his muse, the essence of his creativity, and is repaid in kind. It's an animated metaphor for the process of writing, and many times the characters seem to know they are merely characters in a book. It begins in a hospital where Miles has just recovered, having lost his memory through some accident, but that scenario quickly ends as Erato takes on numerous personalities and attitudes in her interaction with Miles.
This is probably best for those familiar with John Fowles's other works. Mantissa is clever, it's funny, it's self-aware, and it's not going to shake the literary world. It's just a quick afternoon read that gives you a peek into the mind of a writer.