For this case we can translate the sentence given as:
I _______ the book to my cousin.
We must complete the sentence with the most appropriate verb. We know that books are read, so we must find the proper conjugation of the verb "leer":
From the simple past tense we have:
(yo) leí
(tú) leíste
(él) leyó
(nosotros) leímos
(vosotros) leísteis
(ellos) leyeron
So, according to the first person conjugation we have:
Yo le leí el libro a mi primo.
ANswer:
Yo le leí el libro a mi primo.