Something deep that makes you think?
Impossible to pick just one, really, but pretty much all of the candidates would be books of Chassidic spiritual teachings. One that I spend a lot of time with is Netivot Shalom (Pathways of Peace), by Rabbi Shalom Noah Barzovsky (may his memory be a blessing). He was the rebbe (spiritual leader) of the Chassidic community from Slonim, Lithuania (now in Israel), until he left this world about 12 years ago - on the day I was going to meet him! No English translation has been published yet.
"Teaching a Stone to Talk: Expeditions and Encounters" By~Annie Dillard
The Unbearable Lightness of Being by Milan Kundera
the cysteine prophecy
Whats that about?
Whats it about?