I'm not sure specifically as there could be many novels related to billionaires. It might be about the life, rise to wealth, business adventures, or love stories of billionaires.
The main characters in a 'billionare novel' could vary greatly. There may be the billionaire entrepreneur who is the protagonist. Then there could be the shrewd investors or bankers they interact with. There might be a journalist character who is trying to expose the secrets of the billionaire's empire. And not to forget, there could be the friends from their early days who either support or betray them on their journey to billionaire status.
One great billionaire brothers romance novel is 'The Billionaire's Secret Love'. It tells the story of a young woman who gets involved with two billionaire brothers in different ways. The love triangle creates a lot of drama and passion.
A true simp story could be about a guy who always buys his crush expensive gifts without expecting anything in return. He just does it out of pure adoration.
It could be that their storylines and personalities offer an escape or a form of entertainment that resonates with you. Sometimes, we simp for them because they represent something we wish we had or could be.
The power dynamics. Since they are billionaires, there's a certain allure of power and wealth. For example, they can sweep the heroine off her feet with their luxurious lifestyles.
A heartwarming true simp story is that of a guy who would write love letters to his crush every week. He poured his heart out in those letters, talking about how she made his days better. Even when she didn't respond much at first, he kept writing. After a while, she started to look forward to his letters and they eventually became friends and then more. He was just so sincere in his feelings and efforts.