Reviewing this, I find I'm confused anew. From the earlier lesson:

Question

Tu aimes mon pull ? -Oui, il est très beau.
But above:  À qui sont ces livres? Ce sont mes livres.
Since the books under discussion are specific, why would one not say, Ils sont mes livres?

Accepted Answer

Bonjour Susan !

This is the case 1 explained in the following lesson:
https://progress.lawlessfrench.com/revision/grammar/when-to-use-cest-or-il-est-elle-est-to-say-it-is

"1. c'est in sentences "it/he/she is + a/the/my... + thing/people"
If "it/he/she is" is followed by un/une/le/la...(any form of article / determinant) - it is a beautiful dress / she is a nice person - then you will use c'est. "

À bientôt !

Answer

Tnx, as ever, for your help. 
What I'm finding confusing is what appears to be a conflict between the two rules. You say:  mon pull, and il est (because its specific, not general), but you also say, ces livres and ce sont (also specific, not general).  Other than that the first being singular and the second, plural, what is it that makes these two examples different?  
I've read and re-read both lessons on this, and I'm still, unfortunately, missing the point (not that that hasn't happened before).  I hope you will clarify further.