Important for boys:
Digging up the backyard to make a pond, building makeshift shelters out of branches, then hiding inside to spot the wildlife attracted to the pond.

Not important for boys:
Punctuation.

Or did the publisher assume that some kid’s sister would get ahold of The Boy’s Book of Adventure and refuse to let go of it?

Ma: ‘Just let her read from it, Pa.’
Pa: ‘Dammit Ma, that’s Johnny’s book of adventure. It’s the boy’s.’
Ma: ‘Can’t she read it just this once?’
Pa: ‘No way. It’s the boy’s. Says so right on the cover.’

I seriously like the book, and have found it useful to busy idle hands (boys’ and girls’ hands alike). But c’mon! The publishers proof-reader really let the author’s down.