I was actually curious about this myself a while back! 'Cruel Intentions' is one of those stories that lingers in your mind—whether you know it from the 1999 movie or the original source material. The novel it's based on is 'Les Liaisons dangereuses' by Pierre Choslos de Laclos, an 18th-century French epistolary novel. While 'Cruel Intentions' isn't a direct novel itself, the classic it adapts is public domain and widely available. You can find free digital copies of 'Les Liaisons dangereuses' on sites like Project Gutenberg or Wikisource, often in both the original French and English translations.
If you're specifically looking for something with the modern, soapy drama of the movie, though, you might hit a snag. The screenplay was original, so there's no 'Cruel Intentions' novel per se. But diving into 'Les Liaisons dangereuses' is a treat—it’s wild how timeless the themes of manipulation and desire are. The writing style is dense but rewarding, and seeing where the movie drew inspiration adds a whole new layer to the story. Some fan communities might have unofficial novelizations, but for the real deal, the classics are your best free bet.