I found the best way to decode was to write the 2 rows of 26 letters one below the other.
Top row being 25a + real name (ignoring any letters already used + balance of letters in alphabet.
Second row - 16a + balance of letters in alphabet not used.
A letter in the second row must be decoded to it's opposite letter in row 1
For example if you have K in grid - this become D
in the book title. W's stay the same G becomes H and so on.
I do hope you can understand what I'm trying to say.
Tony x