Like Pitlamp,
Definitely caving books, in the local public library, which I stumbled on as a kid.
Main were "Underground Adventure" (Gemmell & Myers) and some kids' adventure stories (by Showell Styles??? ? with titles like 'The River of Adventure'; can anybody identify them?). I also found 1000 Metres Down and Subterranean Climbers, in my opinion two of the best inspirational books written about caving. Funny, I never thought I'd ever go to the Berger at that time!
Then, when I went to secondary school many of the teachers used the (old-fashioned?) method of sitting us in alphabetical order, and I just got plonked next to a kid (12/13 years old) who was already a caver, and he and I became very good friends. I still remember (after far too many years) my first trip, on the BPC winch; and my first 'real' trip, to Ireby Fell Cavern, emerging knackered in what we were pleased to call 'dry grots' ? grotty certainly, but dry ? no way.