Thank you, thank you. Another result of the first book's being so popular was a rash of what you might call "unofficial sequels", of rather poor quality. The second book was written in part as a response to these.
I hope it's not considered too much bad form to guess like that straight after having a question answered. Actually, I couldn't think of any two-part foreign language books of that sort of antiquity, and was about to guess the Bible, although certain that wasn't it, just to eliminate the possibility, when another candidate suddenly struck me.

Anyway, I am now thinking of an ABSTRACT, with ANIMAL and VEGETABLE connections. Game on.

'En un lugar de La Mancha, de cuyo nombre no quiero recordarme...' - I've always liked that as an opening line - it's the 'no quiero' that gives it its interest.

[Animal human] In one connection, specifically yes. In another connection, yes, but not specifically so. In a sense, specifically no.