There are a number of different theories about how these puzzles work. Some of the more high profile ones are Icon to Path (generally attributed to Matt Sparks aka maltedfalcon) and The Shadow Theory (by Deidra and Dustin White). I'm sure there are many more, but these are the two that receive most of the attention.
With only 3 casques found, there are so few data points, it's hard to know if any of these theories carry any real weight. Do your due dilligence and faimiliarize yourself with them, then make your judgments.
Below I will present my approach to attempting to solve the puzzles within The Secret. These are just my opinions, your mileage may vary. That said, I am extremely open to having these opinions viewed critially and challenged through healthy debate.
This section is theories that relate only to the verses.
This section is general theories that relate to ideas that underlie The Secret as a whole.