It took five minutes to write the message, then another hour to encode it, despite its brevity. The method was simple: each letter was replaced with a four-digit code.  The first two digits represented a page number in the book, and the second two were the line number; the letter thus encoded was the first letter of that line.  At the end of each word, they added the letter E, since by design it was the easiest to transmit.  They repeated this until they'd reached the end of the book, and then started again from the beginning.