First Post - What are the Thundercliffe Journals?

The Thundercliffe Journals are a set of old notebooks that I found that contain some extremely puzzling entries.   Because I can't really tell what they mean, I thought that by detailing them in a blog, I could share them with a wider community who might be able to help decipher them.   

There are two hundred of these entries, each of exactly two hundred words each.   Each entry has been encrypted with a very simple letter substitution cipher code, but each entry uses a different combination.   Clearly the ciphers are eventually repeated (there are, after all only 26 letters in the alphabet), but there seems so far to be no pattern to the way it repeats.   As I have not yet deciphered all the entries so far, in fact I'm only up to number ten, there is always a chance that a pattern will emerge.   

I'll fill in more details about how I discovered them and what they look like, which may give a clue to their authorship, as I build up this blog.

Finally, this is my first blog, so please be patient with me.   Any advice will be gratefully received and I welcome any help anyone out there can provide to work out what on earth these journals are about.