There are so many possibilities for a book titled 'under the bridge book' if it's a true story. It could be about the homeless population that often takes shelter under bridges. It could chronicle their daily struggles, their relationships with each other, and how they cope with life on the streets. It might also touch on the efforts of social workers or volunteers who reach out to them in that location.
The song 'Under the Bridge' by Red Hot Chili Peppers was inspired by Anthony Kiedis' real - life experiences. He was struggling with drug addiction at the time. The 'under the bridge' referred to a place in Los Angeles where he used to go to get away from his problems and think. It was a very personal and emotional song for him.
I don't think it is. Usually, such titles are often used for fictional works that draw inspiration from various sources but aren't based on actual events. This could be a case of that.