This song is so well-known that it almost needs no introduction. The everlasting R’n’B hit was written by John Hurley and Ronnie Wilkins and the most popular version is the one sung by Dusty Springfield. Many other artists contributed to making it legendary, like Elvis Presley, Chet Atkins, Joss Stone. The song was originally offered to Aretha Franklin (who is a preacher's daughter), but she turned it down because she thought it was disrespectful.
The song carries a strong blues groove and phrases but it doesn't have a 12 bar form. The first part is in the key of E major with E, A and B7 chords combined. Later, in the second part, it modulates to A major through series of chord changes D7 - A7 - B7 - E7 --- A major (A and D7 chords).
I pretty much stayed true to the original for this song too. It's an exact transcription adapted for the fingerstyle guitar. So you can even play along with the original.