Searching the Internet, I found a published thriller with the same title I plan to use for my own thriller. Can I still use it, or is that illegal?
Much like names, slogans and ideas, titles are not protected by U.S. copyright laws (which is why so many books have the same titles). To qualify for copyright protection, a work needs to possess “a significant amount of original expression”—and while “a significant amount of original expression” isn’t fully defined by hard-and-fast rules, the courts have ruled that expressions as short as book titles do not qualify.
This doesn’t mean that you are free to title your next book Harry Potter and the Sorcerer’s Stone, though. Some titles qualify for trademark protection (specifically, series titles like Chicken Soup for the Soul, Harry Potter, Encyclopedia Brown, etc.). The U.S. Patent and Trademark Office states that a trademark protects words, phrases, symbols or designs identifying the source of the goods or services of one party and distinguishing them from those of others. So once a book becomes successful enough to be considered a recognizable brand, it could be eligible for trademark protection.
Think of it this way: If your book is tentatively titled One Moment in Time—a general phrase that really doesn’t separate it from the pack—and another book shares that name, you’ll likely be OK. But if you were planning to title your thriller The Da Vinci Code—a specific title associated with a bestselling series—you’d better make other arrangements.
As for whether or not it’s a good idea to go with a title that’s already been used—and in the same genre, no less—that’s a question best suited for your editor. However, if you have other title ideas that aren’t already in use, I’d recommend considering a change—if only to alleviate confusion in the marketplace down the road.