Unless the book is encrypted in such a way that it cannot be decrypted without a great deal of effort or the use of computers, it might as well just be written in English. I can read simple substitution cyphers with just a little practice if I know the key--and breaking a simple substitution cypher is literally a child's game. I used to play it with friends in elementary school. If the "code" is really just a simple substitution cypher (e.g., "A" is always represented by the same symbol, "B" by a uniform, distinct symbol, etc.), then it's just English with a different alphabet--no challenge to any computer geek or D&D player. Other encryption methods are a bit more tricky, but unless they are complex enough to actually require a computer to encrypt and decrypt, the method has already been broken a long time ago, and only mere details would need to be worked out.
In short, publishing a "code book" that is a simple cypher or other simple encryption method, in this day and age, is identical to publishing it in plain English.