I'm doing some personal research on this topic - and there are three sources I trust heavily.
Quora, HN and Reddit.
I scraped Quora and made a spreadsheet of their recommendations from the most popular threads about life changing and mind expanding and so on books [1].
[1]: https://docs.google.com/spreadsheet/pub?key=0AnfzjyWu4aYwdGh2amdDTkJzUlktdVlLVXNoZTVDX2c&output=html
Now I'd like to ask HN the same question.