The singer, who recently welcomed his second son with wife Giovanna, made this announcement in a four-minute video titled "Live From The Delivery Room", reported Female First.
"'The Christmasauras' is a story about a boy called William Trundell and a dinosaur, the Christmasauras, and how they meet one Christmas Eve and have a magical adventure," the 30-year-old singer said in the video.
"It's about families and friendships, Santa and sleigh bells, singing elves and flying reindeer, music and magic, and it's about finding your heart's true desire," he added.
Fletcher released his first children's book in 2012, which was co-written with band-mate Dougie Poynter, and it sold over 72,000 copies and became the most popular debut children's novel of the year.