Also check this blog out:
http://funky16corners.wordpress.com/
Almost daily he posts a mp3 from a soul or funk 45", plus a bit about it's history, and a review. And 2-3 times a month he posts a great thematic compilation. This blog can take you from novice to connoisseur.
All great recommendations above. If you already like Ottis Reading, you should check Wilson Pickett, Joe Tex, Don Convey, and also Booker T & the MGs. Definitely Booker T & the MGs, as they were the band backing all of the above. This is the Stax & Atlantic corner of the ring

On the other corner, we have Motown. This year it's its 50th anniversary, and it's a good time to start digging. Last year Motown released a series of compilations with its singles. Each CD comes in a booklet with a history, covers, plus one 45" of one of the songs in the CD.
Of course there is Northern Soul, which has shifted a bit to the mainstream lately (Duffy's Mercy, Pilooski's remix for Beggin', etc).
And there are the new acts. Sharon Jones and the Dap-kings, Nicole Willis and the Soul Investigators, etc.