I don't know the book in question, so these are only guesses:
— "glass walls": his "old" life was like a glass coffin, or a glass tomb: he was "dead", but he didn't know it.
— "dearer": yes, "more precious".
— "unpromised and undeserved": I would assume he means that "no one (i.e. neither man nor God) promised him this new life" and "he was unworthy to receive this new life".
2. It's the Italian for "Jesus Christ".
3. The author likens the patterns of light on the water to a message which he can't read. As the book has some religious content, I would guess that the light is a symbol of his God.
4. I think it means "when you've rested enough".
Let me know if it's still unclear!
Student or Learner