Hmm, and more hmm. Apparently, according to a completely unofficial source, there was a third City book following Ayuamarca and Hell's Horizon (by Darren O'Shaughnessy, aka Darren Shan). "City of the Snakes" is set 10 years after the first two books. Can't find any mention of it on Darren's official site though.
If you ever get the chance to read Ayuamarca and Hell's Horizon, do it :) You might have some luck second hand.