Although the first few books came out regularly, the ninth book, Captain Underpants and the Terrifying Return of Tippy Tinkletrousers (advertised in a teaser at the end of the eighth book), wasn’t released until 2012, after a six-year wait. During this time, Dav Pilkey was caring for his terminally ill father, who died in 2008. In 2009, he signed a deal with Scholastic for four new books, the first of which was The Adventures of Ook and Gluk: Kung-Fu Cavemen from the Future, released August 10, 2010. The second was Super Diaper Baby 2: The Invasion of the Potty Snatchers, released June 28, 2011. The third book, Captain Underpants and the Terrifying Return of Tippy Tinkletrousers, was released August 28, 2012. The fourth book, Captain Underpants and the Revolting Revenge of the Radioactive Robo-Boxers, was released January 16, 2013.