My son loves this book. It has about 50 sections with titles like:
How to set up your headquaters
How to set up an advance warning system
How to create a quick disguise
How to build a camouflaged den
How to shadow a suspect
How to throw an opponent
How to use hand signals while on operation
How to write in invisible ink
How to crack a code
The sections give hints and ideas on various aspects of 'spying', such as code encryption and so on. It is done in a way which is fun, and lighthearted. It doesn't take itself too seriously.
In my opinion, it is most suitable for ages 8-12. Older kids will probably find it a bit simplistic and younger kids will not understand some of it.
It is in the mould of the Dangerous Book for Boys.