Here it is:
“Dear Clayton,
“RTIEZQVLOTKGGDXGNZETHLXLDTIZZTSGZZGFSXYTKQENKTCTW.”
For a moment I took it for a practical joke. Then I remembered a simple cipher which Moore had arranged with the Chief, and I carried the note in to where the Underwood typewriter stood in my study. First, I took a sheet of paper and, counting from the first letter of the keyboard, top row, left, I put down on the paper the numbers, reading left to right and from the top row down, which corresponded to the letters.
Thus the first letter, R, was the fourth letter in the first row. So I put down 4. The next letter, T, was the fifth letter in the first row, so I put down 5. The next letter, I, was 8, E was 3, Z was 20, etc.
Reading in this way, I got:
4, 5, 8, 3, 20, 1, 23, 19, 9, 5, 18, 15, 15, 13, 21, 15, 25, 20, 3, 5, 16, 19, 21, 19, 13, 5, 8, 20, 20, 5, 12, 15, 20, 20, 15, 14, 12, 21, 6, 5, 1, 3, 25, 18, 5, 22, 5, 2.
By an ordinary alphabetic table, which I had prepared, with A as 1, B as 2, C as 3, etc., I found that these numbers represented the following letters:
D, E, H, C, T, A, W, S, I, E, R, O, O, M, U, O, Y, T, C, E, P, S, U, S, M, E, H, T, T, E, L, O, T, T, O, N, L, U, F, E, R, A, C, Y, R, E, V, E, B.
Reading these backward, I got the message:
BEVERYCAREFULNOTTOLETTHEMSUSPECTYOUMOOREISWATCHED.