The main character’s name/what they’re all about (profession, gift, standout personality trait). “The Mansion’s Twins” was simply called “Ellie and Savannah” in my head until I based the title off of their title. Does your main character have a title?
Significant objects. My favorite books are Philip Pullman’s The Golden Compass, The Subtle Knife, and The Amber Spyglass.
Significant place/event: The Golden Compass is also known as Northern Lights.
Special meaningful phrase: Sometimes you don’t quite know what a title means until you get into the story. Just make sure it’s still memorable before readers comprehend the full meaning.
Series Titles: I always knew I’d use “The Mansion’s Family” as a title for something, but I didn’t know if that was the first book, the series as a whole, or something else. Be patient, think things through, and let one idea inspire others. You want your series titles to have some flow to them. Like, “Harry Potter and the fill-in-the-blank.” The Mansion’s Twins is followed by The Mansion’s Family. Dawn’s Acapella is followed by Acapella Angels, and sometime after Sent from a Dream, there will have to be Sent from a Nightmare.
Also consider how easy it is to say out loud. You want people talking about it. "Have you read *insert your title name here*?"
Check your title in Google and Amazon before you fall 100% in love with it. You don’t want something that’s been done before, something that sounds like another popular book. A few similar results might be okay, just make sure you do your research and try to have a title that’s as unique as possible. Otherwise, your book will get lost among similar titles.