The code was formed from highlighted letters from the backcover summary of the book. All together, the result was "asondonttrusthi" which can be spaced out as "ason don't trust hi." It doesn't take a rocket scientist to extrapolate the missing Ms to fully spell out, "Mason, don't trust him."
If this was intentional, then this is the strangest long play coded message in history. No idea if this was intentional or if all of us are just looking for something that really isn't there but it doesn't seem to be accidental. Sadly, I wouldn't be shocked if the graphic designer put this in intentionally but it seems odd that this would be added.
I would like to give a H/T to /u/marciothegreat for pointing this out. And, again, buy Solomon's Builders. It's an excellent read.
What do you think? Comment below.