I've been having a dig around in the books.000 file to get the raw text of the lore books and came across a couple of Thelyron's diaries and a whole text in German. I don't recall seeing these in the game itself and the length of the German text is really unusual. Does anyone know if these texts made it into the game, or were they cut?

The 5 German books are in the easter egg area (Larian/Silver armour-screenie-spoiler - broken screenshot links, though). I'm not sure about Thelyron's diaries.

Translation of the "Tales of the Lost Kingdom : Rescuing the Princess"

The History of the Lost Kingdom books

Now that you have mentioned the silver armour location it rings a vague bell, It's been well over a decade since I last went there (probably should have finished my new run through before digging through the game files).

Reading through the story, adding that in was a very Larian thing to do, nice to see they haven't changed at all on that front in the last 20 or so years.

I'm fairly certain on the diaries not being in there, I've been over the catacombs with a fine tooth comb and that would be the most logical place for them, or the pauper's graveyard..... Unless there is another location linked to Thelyron I've forgotten about.