I-Spy books were small paperback volumes that were popular in the 1950s and 1960s. Each book covered a subject such as I-SPY Cars, I-SPY on the Pavement, I-SPY on a Train Journey, and so on and so on.
The object was to be vigilant and spot objects such as animals, trees, policemen, fire engines, sea shells etc. etc. and they were recorded in the relevant book, and this gained points. More points were available for the more difficult spots. Once you had spotted everything and the book was complete, it could be sent to Big Chief I-SPY for a feather and order of merit.