That alone makes Cleckner's book worse than useless IMO because it just perpetuates the stupidity that we see here all the time when uneducated shooters show up to argue that MOA is better because they think in inches or some such other idiocy.
So, Cleckner's book isn't bad at explaining what a Minute of Angle is... or what a Mil actually is... or that they can be used to measure within the context of being angular units of measurement, and I actually find that the diagrams are quite useful in how he explains
just the concept that constitutes how MOA and Mils are derived (found on pages 128-136). What gets stupidly confusing in his book is the flip-flopping between angular and linear units of measurement in his examples. I think it really should have been structured so that, unless a linear unit of measurement happened to be pertinent or important, every conversation about something that can be measured in an angular unit of measurement was given in MOA or Mil values, without any conversion to linear (inches/cm etc.) present. Do I think that certain things in his book could be written differently and probably improved? Definitely, but that's also a subjective answer to a question, coming from a sample size of one person.
On the other hand, Frank's book does a fantastic job at explaining something which is beaten to a bloody pulp in the forums here and elsewhere (and he does it on pages 161-165 of his book), and succinctly makes the concept operational for someone behind the rifle. He summarizes and distills what works, regardless of the preferred angular unit, because he is teaching not only each language (MOA/Mil), but also how to translate between them quickly to work through "issues" without even needing to touch a linear unit of measurement. I also really like the way that the book gives a dirt simple solution to linear units of measurement by encouraging the use of the reticle as a ruler to measure what you see, and not overcomplicating the concept.
If you compare the books to each other, they are apples to oranges in many ways. Cleckner's book is really aimed at someone who knows very little about precision rifle shooting; for people who have a background already, there won't really be much that is magically revealed as an "ah-ha!" moment. There are some good (but basic) gems though. Frank's book has gems of knowledge for people of all skill levels, and it's a relatively quick, enjoyable read that will benefit a total novice and also those of us who have had a bit more exposure to shooting rifles at distance - this is where Frank's book really shines, and one of the reasons that I hope that he writes and releases more books to compliment this one.