The book turned out to be The Haunting of Hill House, a 1959 novel by Shirley Jackson, and Tennant grabbed it from the shelf just in a knick of time.
Inside it was a sheet of paper with the lyrics for the band's new song, A Sky Full of Stars.
Library staffer Harley Couper, who had been following the tweets and twigged just minutes after Tennant arrived at the library, raced downstairs to find the book.
"I couldn't find it and then I saw it down on the ground it was on the bottom shelf, and obviously someone had pillaged the letter from the book mere minutes before me," Couper said.
"Good on her, well done, but I would have loved to have found it myself."
Ms Bracken said the minutes leading up to the big find had been tense, as others began to tweet the name of the book.
"Then Annalie called me and said, 'I found it'. I said, 'are you serious?' and she said, 'Yeah, I got it'. I was very, very excited."
Ms Bracken is driving home today to meet her friend and their newfound souvenir, which she has no intention of selling.
Coldplay has congratulated the girls on Twitter.
Here's how the hunt played out on Twitter:
CLUE 7: Where the Apterygidae family have plenty in the bay, find the City Library on Willow and look for Ms Jackson's THoHH #lyricshunt
