The I-SPY books are spotters' guides written for British children, and particularly successful in the 1950s and 1960s.
The I-SPY Tribe was based on the I-SPY Books, some forty small volumes that sold in hundreds of thousands. Each book covered a subject such as I-SPY Cars, I-SPY on the Pavement, I-SPY Churches, I-SPY on a Train Journey, etc. As children spotted objects such as coalhole covers, oak trees, semaphore signals, fire engines, whelks, and so on, they recorded the event in the relevant book, and gained points. Once the book was complete, it could be sent to Big Chief I-SPY for a feather and order of merit.
This version takes it a step further as only Wizards could actually fill in the book, and presumably they'd be foolish to take their eyes off whichever demon they'd called up in order so to do, but the idea is instantly recognisable to British readers.