If it is simply books about soul music, try Chicago Soul - Robert Pruter, The sound of Philadelphia by Tony Cummings, Making Tracks by Charlie Gillett (the Atlantic story) or Sweet Soul Music by Peter Guralnick, if you do not already have them.
Ladies of Soul by David Freeland is a good read and having read this one, enjoying the piece on Bettye Lavette, I bought her book A Woman Like Me - what a story. This woman tried everything!
Tamla Motown - the stories behind the singles, is a good coffee table book, while I also enjoyed Berry, Motown and Me by Ranoma Gordy Singleton.
Books on the northern soul scene are numerous, some better than others and some getting more stick than others, but I found The Manchester Wheelers one of the better ones.
Like the music itself, it isall a matter of taste.