I know this sounds snob, but… After reading all kind of books, a lot of them often recommended here, If you want to actually find challenging books I recommend sticking with essay, textbooks (go to a university page and buy the textbooks listed for a course), and classics (stuff like Hume, not How to influence people), and avoid anything you can find in an airport bookshop. But careful, they are actually challenging, and won’t be as satisfying as reading some divulgation or self-help book, and will require actual effort.
I love to read books like How we sleep, or black swan, etc. but personally I think they make me feel good/smart, more than they challenge me.
>a way to think more intelligently about questions large or small
All Philosophy books are meta