[sergykid]

i found 3 books like this but i still don't get it

what's the big idea with them? and if there are more where are they

[Onisuzume]

The page on the right contains a hint on where to find it.
That's part of the fun.

So checking book 3, that says...
The smart fishing also tries his luck on the other side of the lake.
So grab your map and check for lakes.