I thought it was then and I still think it is now:
DAN COATS
The "Anonymous" Amazon stub referenced NCAA baskeetball, which Coats loves (IU and IUPUI, the latter which he attended for his JD; and Indy is home to the NCAA), and the book stub was set in "Coaching", which Coats did a bit in his younger days.
Well, that presents a problem, if it turns out to be Dan, because Pence will have to 100% tear Dan down in the media and enlist the delegation & Mitch Daniels to help do it, because Coats won't respond directly (and no longer can use his buddy RINO mouthpiece Dick Lugar, cause he checked out in April).