I wrote a book and after publishing it, went to search for its availability. That's when I discovered that another book written 8 years ago has the same title. My title, unlike the other one, includes a subtitle. Will that help me avoid legal issues, or should I just change the title? Same subject matter, but written in different style, I'm sure.
I'm not a lawyer, but I found an article on the web published by Writer's Digest (who, I assume, are knowledgeable) that says that a title is not copyrighteable, so you can give your book the same title as an already existing book, unless the title or some part of it is a trademark (such as "Harry Potter") or so unique and well-known that it has brand-like quality (such as "The Da Vinchi Code"). The more your title resembles a common phrase (such as "Green Grass"), the safer it will be to reuse.
Like the Writer's Digest article, I would like to point out to you that having a book in the same genre with the same title can hurt your sales and your brand as an author (for example, because the other book is bad and readers will confuse your book with that one and not buy it, or they might search for your book and buy the other one instead, because its by Stephen King while you seem like a rip-off, etc.).
Why didn't you research the title before you published the book? That seems like one of the first things you should do.