GCHQ Puzzle Book

GCHQ, the top-secret intelligence and security agency, is opening up its archives of decades' worth of codes, puzzles and challenges for everyone to try. Over the years, their codebreakers have helped keep our country safe, from the Bletchley Park breakthroughs of WWII to the modern-day threat of cyberattack. Whether they're recruiting new staff or challenging each other to the toughest Christmas quizzes and treasure hunts imaginable, puzzles are at the heart of what GCHQ does.  This book includes tips to help you break codes, and a range of puzzles at varying levels of difficulty, for example:
If 3=T, 4=S, 5=P, 6=H, 7=H ... what is 8?