Puzzle is an illusion. Only 10 letters on the diagonal needing changed, if you've researched on the author you've spotted it and decoding becomes encoding. That is if you can call adding (numeric value - 1) of a letter to the key letter 'encoding'. For anyone still unsure of the extra words, 7 to 13, 11 to 30, 12 to 22, 13 to 32, 15 to 42, 16 to 45, 18 to 29, 20 to 40, 22 to 46, 29 to 18, 30 to 16, 32 to 11, 38 to 15, 40 to 20, 42 to 38, 44 to 12, 45 to 7, 46 to 44 (I hope I've got all the moves the right way round).