The UK’s Government Communications Headquarters (GCHQ) has, over the last few years, published puzzles that keen members of the public could solve to get noticed by the organization’s recruitment team. What they didn’t let on, though, is that what we’ve seen until now is a mere glimpse of a giant puzzle archive going back decades.

Internally, GCHQ employees have been designing and setting each other puzzles since the 1980s. The mindbenders first took place over the Christmas period, evolving to include an Easter Teaser and even a real-life Treasure Hunt, where about 50 staff descend on a Cotswold town. The GCHQ Puzzle Book is a chunky compendium where readers are introduced to the whole back catalogue, which has remained secret until now.

One puzzle type that stands out as being particularly “meta” is the Puzzle Hunt – a set of pictorial puzzles that don’t come with any questions, so that the solver has to first work out what to do for each part, then after solving them, combine the answers to solve a final puzzle, which itself has no question. Tantalizing too is the “artwork” on the book’s inside cover, which is a collection of higgledy-piggledy letters. Together with the advice that “there may be more questions in the book than those which are immediately obvious”, the artwork looks suspiciously code-like.

There is also an entirely fresh “competition” puzzle to be solved; get in quick as the deadline to enter is 28 February 2017. Keen-eyed readers will spot that Physics World gets a mention. That’s because in 2013, for the 25th-anniversary issue of Physics World, we worked with GCHQ to produce a set of physics-themed puzzles, the first of which is included in the book’s introduction.

• 2016 Penguin Random House £12.99 336pp