I stumbled across this book, and caught puzzled by the very unusual subject:
the (true) story of an intimate friendship between a student and a teacher
who kept a lifelong correspondence. Unusual in all this is that rarely is "Life" itself the subject of the letters: the interaction being in fact based on mathematical problems that both student and teacher shared a passion for.

Mathematics appearing to the eyes of the most as a cold, emotionless subject, one may fear that the book itself comes out as a cold, emotionless collection of complicated formulas.

The contrary is true. I found in the book gems of magnificent humanity. Going through the pages, reading those mathematical letters one finds, here and there, delicate (and sometimes touchingly shy) personal remarks which in fact showed how much the writers cared for each other. Mathematics, it becomes soon apparent, is just a bond connecting two soul mates, and "teacher" and "student", perhaps, may be even more appropriately interpreted as "father" and "son".