Corinna said: “I would like to sincerely thank you for your message and nice gift that will help us through this difficult time.
“It is good to receive so many kind wishes and other well-intentioned words - which is a great support for our family.
“We all know Michael is a fighter and will not give up.”
The letter was written as a response to Hamburg bandleader Sascha Herchenbach, who sent the family a recording of a new song Born To Fight.
Mr Herchenbach revealed the contents of the letter in an interview with German magazine Bunte last week.
He told the publication: “I had not expected to get an answer and was overwhelmed.
“The letter was handwritten and signed by Corinna on her stationery.
“She wrote that she was very thankful for the gift and helped her and her family over this difficult time.”
The Schumacher family rarely comment on Michael's health, and it's believed he is being cared for by a team of doctors at his home near Lake Geneva in Switzerland.