Wait, are you talking about 'The Insider' by Stephen Frey? That's the first thing my brain goes to. If so, that one's a financial thriller, entirely fictional, about a guy who uncovers a conspiracy at a Wall Street investment bank. It's a fun, fast read, but no, not based on a true story. It's pure corporate espionage fantasy.
If you mean something else called 'Insider', maybe by a different author, the title's pretty common. I'd check the author and the book's description on a site like Goodreads. True-crime or exposé-style books often have subtitles like 'A True Story' or mention the real event. A purely fictional thriller usually doesn't. Without a specific author, it's hard to say for sure, but my guess is most novels with that title are works of fiction.