Is it permissible for him to sell political books and plays?

The basic principle concerning selling is that it is
permissible because Allaah says (interpretation of the meaning): 

“Allaah has permitted
trading and forbidden Ribaa”

[al-Baqarah 2:275]

al-Jassaas said: This is
general in meaning and indicates that all transactions are permissible. 

Ahkaam al-Qur’aan, 2/189 

This is as far as the
general principle is concerned. But if the item sold is haraam, or if it is
usually used for haraam purposes, then it is not permissible to deal in it,
either buying or selling. Based on that, one of the following must apply to
these books that are mentioned in your question: 

Either they are useful books which will be of benefit to the
reader even if they are not about religious topics, such as history,
politics, physics, computer science, etc and do not include – as far as you
know – anything haraam, such as lies or promotion of evil or anything that
would deceive and mislead people. These come under the principle mentioned
above, which is that it is permissible to sell them according to sharee’ah.
If it is permissible to sell them then it is halaal for you to use the money
you get for your spiritual and worldly interests. 

Or they are books that are haraam in and of themselves and
are harmful to the reader because they include lies, distortions of the
facts and of religious truths, corruption of morals, religious commitment
and beliefs, or changing the meaning of modesty in women and chivalry in men
– as is the case with most novels and plays, whether Arabic or foreign. It
is haraam to sell these kinds of books; rather they must be destroyed and it
is not permissible to make use of the price, because the Prophet (peace
and blessings of Allaah be upon him) said: “When Allaah forbids a thing to
people, He also forbids its price.” Narrated by Ahmad, 2956; classed as
saheeh by al-Albaani in Ghaayat al-Maraam, 318. 

And Allaah knows
best.