Maths biography. love letters explaining maths, a condition imposed by the father. what is the book called?

The book is written by the daugher of the mathematician. her parents met on a train. there was quite an age differnce which led the grandfather to impose a courtship/corresponce where by his daughter would be tutored in maths. whta is the book called?
Answer
"mathematics with love"http://www.iee.org/oncomms/circuit/benefits/editorials/bookreviews/mathematics.cfm