To get really rich your book or series of books has to be made into a film.
Give it a go - many thousands do, and get published.
It's best to have a somewhat original plot...Here's two you couldn't make up...
A Northern labour ! MP known for his deeds of darkness and a public school educated posh pratt with ambitions to be a chancellor accidentally meet on holiday. They meet on a yacht, owned by the head of the richest banking and oil family on the planet, discretely anchored off a lesser known sunny island. The title could be:
'THE PRINCE THE OIK AND THE BANKER'.
A London banker and a New York banker get together to buy a huge luxury yacht - you could call it:
'A SALE OF TWO TITTIES'.
The truth is indeed stranger than fiction.
Just remember that very few authors make a living out of writing.