I read "The Inner Game of Music" ages ago and found it helpful to my choral singing at the time. It's probably been revised since I read it, but more than one of the reviews on Goodreads suggest that reading the original one on Tennis might be preferable. (For one, "If you're looking for similar concepts I would highly recommend reading 'the Inner Game of Tennis'. It presents the same concepts but in a more concise manner that does not assume the reader is clueless. The tennis metaphors are not a hindrance and the book is considerably shorter!" https://www.goodreads.com/book/show/848522.The_Inner_Game_of...)