Read the Book of Knowledge from Vedia Bülent (Onsu) Çorak

With regard to German literature, I'd recommend Adalbert Stifter's Bunte Steine. Also, Dostoevsky's Crime and Punishment, although not German literature, is a fine psychological and existentialist novel as well.

I recommend "Oh! A Mystery of Mono No Aware" by Todd Shimoda.  It was so deep and philosophical, by talking about it to friends, I ended up getting three of them to read it too.  In addition to it being a beautifully-made book, it is a captivating story and study into human emotions.  Mono no aware is an old Japanese poetry term that is very difficult to define.  Throughout the book they attempt (between every two chapters) to explain some of the concepts.  Meanwhile the main character searches for emotion and feeling in Japan.  He feels nothing in our empty society, emotionless, and this quest leads him into the Japanese suicide clubs.  His former student and mentor tries to save him. Poignant, deep, truly a moving book.