Beginning in 1936, Art Shefte produced an amazing set of books simply named, Piano Improvising. While not specifically for sacred music, he described it as "A positive system showing how to convert popular songs from their printed form into modern professional style." These two books contain one of the best explanations of left hand technique in chording that I have ever seen.
Can you buy these two volumes today? In a word, No. And Yes. While not in print, they can still be found in places such as second hand book stores. The reason I tell you about them is so you will know to look for them. I first heard of these books on an e-mail list I subscribe to, and began watching for them. I then stumbled upon both volumes on eBay (two separate auctions, a few weeks apart, from separate sellers!) and bought both. Keep your eyes peeled!
64 pages each. By Art Shefte. Vol. 1 Copyright 1936. Vol. 2 Copyright 1938. Forster Music Publishing, Inc., Chicago, Ill.