These sentences are in the indicative mood, which means the direct object pronoun will come directly before the verb in the sentence.
The direct object pronoun for the object los libros in the first sentence is los. Remember, the direct object pronoun comes before the verb, so the sentence becomes:
No los leas.
"You don't read them."
The direct object pronoun for the singular object el libro in the second sentence is lo. Following the same rule as above, the sentence becomes:
No lo lea.
"He/She/You (formal) don't read it."