These books are from my personal collection. The two hardcovers are from the late sixties:
This original art for ZACHERLEY'S MIDNIGHT SNACKS was sold by Heritage Auctions a few years ago for $7170.00.
The Tarzan series of books were done for Ballantine in the mid-sixties. They don't strike me as Powers-style, although they are attractive in their own right. I wonder how well they sold. I think Edgar Rice Burroughs fans wanted covers more like the Ace printings of public domain books by ERB, with covers by Frank Frazetta and Roy G. Krenkel.
The oldest book I have with credit for Powers is this 1949 Comet Books original paperback. He did both covers and interior illustrations. You can tell by the interiors that Powers was an artist who was as at home with traditional action illustrations as abstracts. The smaller illustrations show what his later work had a whole lot of, a very strong sense of design with an abstract feel.
Added January 18, 2014:
Two more paperbacks I found after posting the above covers.
I have a copy of BEYOND #1 from 1953 in my collection, but it's in terrible shape. I felt fortunate to find an excellent cover online. Thanks to the scanner who posted this on Flickr.