a second book in the series. Cunningham, while working for Bloomsbury in London, famously signed up J. K. Rowling, and this connection led to the book being branded "the next Harry Potter". The authors and Barry Cunningham also decided to retitle the book Tunnels, to reflect that it had been changed by some limited editing. With the announcement of the publication date, and press coverage in the UK, the price of the original self-published books jumped dramatically, with one copy selling for £950. Tunnels was released in the UK as a softcover on 2 July 2007, and in the United States as a hardcover on 10 December 2007, and as a paperback on 1 February 2009. In Canada, the book was released as a paperback on 7 July 2007, as a hardcover on 1 January 2008, and a mass market paperback on 1 February 2009. In the United States, Tunnels had an initial printing of 100,000 copies. In February and March 2008 it appeared on The New York Times Children's Chapter Books Best Seller List. The sequel, Deeper, was released in the UK on 5 May 2008 (in the United States on 3 February 2009), and a third book, Freefall, was published in the UK on 18 May 2009 (in the United States on 1 February 2010). The fourth book, called Closer, was published in the UK in May 2010, and the fifth, Spiral, was released on 1 September 2011, the authors released the final novel, Terminal, in 2013, concluding the series as a hexalogy. Critical reception Many reviewers criticized the first third of Tunnels for its slow pace, but praised the remainder of the book for its fast-paced excitement, suspense, and adventure. In Britain, children's author Philip Ardagh, reviewing for The Guardian, thought the long wait for Will to discover the underground city could dull the reader's anticipation, noting that the event did not occur until page 170. He did observe, however, that when the city is reached, "fantastic fun" begins and that from then on its well-paced, exciting and – in places – frightening and bloody." He thought the characters "splendidly named and drawn". In The Sunday Times of 7 July 2007, Nicolette Jones described the book as "a good adventure yarn ... [b]ut after 460 suspenseful pages it is frustratingly inconclusive." She noted the book became a best-seller the month of its release based simply on "stories