Here is a link to a thorough description and discussion of Spieth's grip,
http://golf-info-guide.com/golf-grip/jordan-spieth-grip/
---- "Unlike many golf phenoms, this precocious Texan doesn’t boast textbook form. In fact, “unorthodox” would be a kind way of describing Jordan Spieth's grip.
Practically all accomplished golfers, from pros to the club level, match their left and right hands on the handle. In other words, the palms face each other directly regardless of their position (neutral, weak or strong).
Not Spieth’s. His left hand is in a relatively weak position, or rotated toward the target, while his right is somewhat strong, or rotated away from the target. This causes Spieth’s left arm to bend slightly while his left wrist bows outward at the top of the backswing.
It’s neither conventional nor pretty, but it certainly works. Still, many observers believe Jordan Spieth will have to alter his grip at some point to ensure long-term success." ----
In the Stack and Tilt book on page 52, it says : "The right hand grips with the fingers and covers the left thumb".