I don't think math books make you smart or see the world through a different angle (note : I studied maths a university so I might be completely biased ).

I'd say books about fields you don't necessarily know much about (if you're a web developer, it can mean both books about linux kernel programming and cooking books) including the ones that you think are about boring topics. They will help you understand that you don't know much about anything, and that there's usually several ways to look at something (say your preconceptions vs what you learn from the book) I think this drastically improves your critical thinking.