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.
Let him yt stole steale no moare but let him rather laboure with his hondes some good thinge that he maye have to geve vnto him that nedeth.Ephesians 4:28 TYN