Answer:
This is Shakespeare's Sonnet 45.
In this sonnet, he finds himself in continous fluctuation between joy and sorrow. This is as a result of the absence of the one he loved. His thoughts were always moving to and fro between him and his love.
Actually, this sonnet relates to his previous sonnet. He finds that his thoughts and desires are not so much in himself, as with his beloved (hence present-absent.)
Substances were said to be made up of fire, air, earth and water. But when a substance is deprived of two of them, air and fire, (the other two) which correspond to thought and desire, the body responds and sinks into melancholy and decay.
Them would be the correct pronoun for this sentence because books is plural and is an object, so its pronoun must be third person. The 3rd person plural pronoun in English is they, and its objective/accusative form (the form used when the item is the object of a verb) is them, so there's your answer.
