I got a visit when I was young (planned, agreed to in advance) from an officer from my countries national intelligence service. I was a reference for someone going to work for the equivalent of the NSA.

He spent a lot of time looking at my mum's (questionable) books and making notes.

I've thought about that a lot over the last 20 years. How much insight you can gain from someones bookshelf.

I'm now addicted to it, where ever I go. Feels rude and invasive though, so I try not to be too obvious