"The second book is mine."
Translation:La dua libro estas la mia.
Do we need la in la mia? It sounds awkward to me to have a definite article before the pronoun.
I explain it not with a book but with a car:
"tiu aŭto estas la mia" makes it clear, that this car is your (in the context) only car.
"tiu aŭto estas mia" could also mean, that also other cars are yours.
I had this on a multiple choice question just now. The system was looking for two correct answers - one with "mia" and one with "la mia." I imagine that could be frustrating for a learner.
jomue's answer is a good one.
"The second book is the mine"? If that second 'the' is optional, I would like to have the option to not be forced to use it.
I tend to translate "mine" as "la mia", and "my" as "mia". So "Tiu libro estas la mia" or "Tiu estas mia libro".