This might not be a very specific question, but I was wondering this morning. If one were to encode a message by making a string out of an entire book and writing the message by searching for a random occurrence in the "book-string" of each letter in the message and replacing it with the string index of the letter. Additionally you could make sure no number appears twice in the message as there is normally more than one occurrence of each letter in a book. This way no "letter" in the encrypted message would repeat.
How safe is this, assuming you passed on the key safely, and how would you go about breaking it?
Thanks in advance :D