Annotating the world

What is the best way to annotate the world for other people?. I think about this a lot. There’s so much room for improvement in how the web could help with this. I want to see the notes my friends (/ friends of friends) made when reading the book I’m currently reading. Or a list of places they went and notes they took on the city I’m about to visit. Twitter is ok for this but searching isn’t great. The ideal would be if everyone had personal sites where they documented this stuff. But again searching them is tricky.