At the close of the 1962–63 season, Allison received an offer to coach Canadian team Toronto City over the summer and took Book with him. Though Allison left after a short time to take up a position at Plymouth Argyle, Book stayed three months, in which time he was voted the best full-back in Canada.
Upon his return to England, Book was signed for Plymouth by Allison for a fee of £1,500, and Book entered the Football League for the first time at the age of 30, though Plymouth believed him to be 28 – Allison had advised Book to doctor his birth certificate as he thought the Plymouth board would not pay £1,500 for a 30-year-old. After making 81 league appearances, Book followed Malcolm Allison again to Manchester City two years later, this time for a transfer fee of £17,000. Manager Joe Mercer was initially reluctant to spend such a fee on a player over 30 years old, but was persuaded after Allison pointed out that Mercer’s career had included a successful move at a similar age, when he joined Arsenal from Everton aged 32.
Book prospered under the management of Mercer and Allison. He made his Manchester City debut in the opening match of the 1966–67 season, a 1–1 draw with Southampton, and became a near-permanent fixture in the team. In his first season at the club he missed just one game, becoming the inaugural winner of the club’s Player of the Year award in a season in which the club consolidated their position following promotion.
In the 1967 close season, Book was named captain following the transfer of previous captain Johnny Crossan to Middlesbrough, and was henceforth nicknamed Skip by his teammates. His first season as captain was a very successful one, leading Manchester City to their second league championship and playing every game. An Achilles injury sidelined Book for the first four months of the 1968–69 season, but he returned to the team in time for the start of their FA Cup run. In the week preceding the cup final, Book was named the 1969 Football Writers’ Association Footballer of the Year Award, sharing the accolade with Dave Mackay. The following Saturday Manchester City played Leicester in the FA Cup final. Manchester City won 1–0, and captain Book lifted the trophy. The following season City became the first English team to win a European and domestic trophy in the same season, the European Cup Winners’ Cup and the League Cup. Book retired from playing in 1974, passing the captaincy to Colin Bell. He made 242 football league appearances for the club, and for many years was City’s most successful captain in terms of trophies won.