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.