This book is recommended by Douglas R. Hofstadter in Gödel, Escher, Bach:
For some delightful anecdotes about Bach’s extemporization, see The Bach Reader, by H. T. David and A. Mendel.
The book can be borrowed from the Internet Archive.
And whosoever shall compel thee to go a mile, go with him twain.Matthew 5:41 KJV