If you cracked Challenge 6A then you may be wondering about the reference to The Copperfield Disclosures. Charles Babbage was a good friend of Dickens, so would have known that the book that ended up published as David Copperfield had a lot of possible titles. Dickens wrestled with this for a while and even wrote to his friend John Forster to ask which he preferred. That list included the title mentioned by Ada.
Of course just knowing which book was meant is not enough. There are a lot of ways a book can be used to encrypt a message, including the Book Cipher itself, but in this case Ada and Charles are using the novel to exchange a key. Presumably it worked like this: Charles and Ada had already agreed a sequence of numbers, one for each day or each message, and the correct number is paired with a book mentioned in their letters so that the next cipher key is taken from the title of that numbered chapter.
You can take a look at the case files to see how it worked in this case.
Good luck, Jodie
PS, did you like my reference to the keyword in the Challenge title?