A couple of books, first The Man Who Wrote Frankenstein[1] and later Shelley Unbound: Discovering Frankenstein's True Creator[2], argue that Frankenstein was actually written by Mary Shelly's husband, Percy Shelly.

[1] - https://www.amazon.com/Man-Who-Wrote-Frankenstein/dp/0943742...

[2] - https://www.amazon.com/Shelley-Unbound-Discovering-Frankenst...

There's a book called The Man Who Wrote Frankenstein that argues that Frankenstein was actually written by Mary Shelly's husband, Percy Shelly.[1]

A later book, Shelley Unbound: Discovering Frankenstein's True Creator makes the same case.[2]

[1] - https://www.amazon.com/Man-Who-Wrote-Frankenstein/dp/0943742...

[2] - https://www.amazon.com/Shelley-Unbound-Discovering-Frankenst...