Donny Hathaway, like Sam Cooke, died way too soon.
The Ghetto
Jealous Guy
The Staples Singers are generally considered more gospel but I think a lot of the stuff they did falls under the "soul" heading. Mavis still performs, I saw her a couple of years ago and she was awesome.
Respect Yourself
Let's Do It Again
Also ElD mentioned Bill Withers, excellent choice. Imo his "Live at Carnegie Hall" record is a must have, one the best live albums of all time