J'ai plus de livres que toi: the "s" in plus is pronounced.

Bonjour Susan ! Yes, it is correct, as "plus" is here used in a positive sense (meaning "more"), and not as a negation ("not... anymore"), you therefore need to pronounce the "s" (not all the time mind you, there are exceptions, but not there!) :)

Bonjour Aurélie, This is good to know. Merci!