Okay so I gotta say, most CEO romances aren't exactly famous for their slow-burn. They tend to jump right into the forced proximity or the contract marriage. But if you're hunting for ones where the tension actually simmers, 'The Love Hypothesis' is basically the poster child. Even though the hero is a professor, not a CEO, the dynamic is super similar—aloof, brilliant, emotionally closed-off genius. The way the fake dating builds is agonizing in the best way.
For a proper CEO, Tessa Bailey's 'Fix Her Up' has a subplot with the heroine's brother, Travis, who's a CEO type. The main couple isn't slow-burn, but his side story with Georgie's friend has this years-long pining thing that really delivers. It's a quieter burn amidst all the usual Bailey spice. Sometimes you gotta look for the secondary characters in these books to find the real drawn-out angst.