Because of this thread, I searched for and ordered a copy of Bluegrass, Belles and Bourbon
. I haven't looked at the book since I researched the documentary back in 1991-92. I didn't care for it very much because I found a lot of it inaccurate, so my answer to the observation about McKenna's distillation proof is that it probably was simply wrong.
The best book I know about barrel-making (cooperage) is The Cooper and His Trade
, by Kenneth Kilby, which is readily available.
My list of recommended bourbon and related books can be found here