Charlie Brown is reading The Gulistan of Sa'adi.

The Gulistan (Persian: گلستان‎ Golestȃn “The Rose Garden”) is a landmark of Persian literature, perhaps its single most influential work of prose. Written in 1259 CE, it is one of two major works of the Persian poet Sa'di, considered one of the greatest medieval Persian poets. It is also one of his most popular books, and has proved deeply influential in the West as well as the East. The Gulistan is a collection of poems and stories…. It is widely quoted as a source of wisdom. …  [T]here is a common saying in Persian, “Each word of Sa'di has seventy-two meanings,” and the stories, alongside their entertainment value and practical and moral dimension… are said to contain sufi teachings.

Full text of The Gulistan is available for free online at many sites, for example, Internet History Sourcebooks Project.

The Gulistan itself is quite readable. This is the story that opens the book:

I have heard of a king who made the sign to put a captive to death. The poor wretch, in that state of desperation, began to abuse the king in the dialect which he spoke, and to revile him with asperity…. The king asked, saying, “What does he say?” One of the Viziers… made answer, “O my lord! he is expressing himself and saying, ‘Paradise is for such as are restraining their anger and forgiving their fellow-creatures; and God will befriend the benevolent.’ ” The king felt compassion for him, and desisted from shedding his blood.

Another nobleman, and the rival of that former, said, “It is indecorous for such peers, as we are, to use any language but that of truth in the presence of kings; this man abused his majesty, and spoke what was unworthy of him.” The king turned away indignant at this remark, and replied, “I was better pleased with his falsehood than with this truth that you have told; for that bore the face of good policy, and this was founded in malignity; and the intelligent have said, ‘A peace-mingling falsehood is preferable to a mischief stirring truth’: Whatever prince may do that which he (his counselor) will recommend, it must be a subject of regret if he shall advise aught but good.”

It contains many wonderful aphorisms that are quoted in the context of longer stories, for example:

Ten dervishes can sleep on one rug, but two kings can not be accommodated in a whole kingdom.

In the 19th century, at least according to Edward Eastwick, The Gulistan was well known by Europeans:

The Gulistan of Sadi has attained a popularity in the East which, perhaps, has never been reached by any European work in this Western world. The school-boy lisps out his first lessons in it; the man of learning quotes it; and a vast number of its expressions have become proverbial.
-Edward B. Eastwick (1880)

And in 2009, President Obama cited The Gulistan in a video message to Iranians in honor of Norwuz:

(Illustration: Peanuts, 2/10/1964, ©United Feature Syndicate.)