The Screwtape Letters by C. S. Lewis

Even if you’re not religious (I’m barely so), there’s a lot in it that shows you how we often work against ourselves, our own interests, and our own happiness. Since reading it, I constantly find myself getting agitated, or selfish, or whatever other negative emotion, and I’ll remember something Screwtape said and see my own behavior/emotion in a more objective light. It really has quite a lot of “if you’re unhappy, you’re probably doing it to yourself” advice.

In case anyone is unaware, the book is letters from a senior demon to a junior demon on how best to tempt and torment “the patient”. Since they can’t DIRECTLY influence us, they use thoughts and internal monologues that they can influence in order to alter our behavior. For example, if you want to do something really good for yourself, your internal voice might say, “Yes, but I’m hungry now, and this is far too important to do on an empty stomach… so I’ll get some lunch first and THEN get started on this great thing” and then you have lunch, get satisfied, lose momentum, and never do the thing (that might have changed your life!).

The John Cleese narrated audiobook version is gold.

Don Quixote. Long, funny, interesting, you could get it in either/both languages (if you speak Spanish fluently/natively, then that’s a no brainer).