The correspondence between symbols and letters becomes clear when you add a box around each symbol:
I like to think of many of these letters as showing what would happen if you took a normal block capital letter and puffed it up (or baked it like a cookie to make it spread out) inside a constricting box, causing all the holes and gaps in the letters to squeeze together into dots and lines.
Escher's signature provided the M, the C, and the E; David, Vic, and David created the rest of the letters. I'd like to see the unboxed letters turned into a monospace font; if anyone knows of such a font already existing, please point me to it.
I'm not terribly fond of the J, the P, and the R, and I have qualms about the K, the Q, and the X. If you have ideas for better versions that don't include enough lines to make the unboxed shape completely obvious, let me know.
By the way, the secret message reads "THIS IS A SECRET MESSAGE" (with a line break in the middle of the last word).
If this column weren't already late, I would go on to discuss figure/ground illusions, and fonts consisting of the shadows of letters, and the ways that invisible boundaries can define a space, and would point you to some of Douglas Hofstadter's discussions of letterform recognition. But as it is, I'll leave you with just a set of interesting letters.