(the Peak of Eloquence) is a collection of sayings and writings of Imam 'Ali (a) which have been compiled by al-Sayyid al-Radi in late 4th/10th century. The criterion for selecting the sayings has been the eloquence of speech. Nahj al-Balagha has been organized into three parts: sermons, letters and short sayings. Throughout the sermons, Imam (a) calls people to obey God's orders and abandon prohibitions and in his letters advises his agents to observe the rights of people. Short sayings of Nahj al-Balagha are a collection of wise sayings stated at the Peak of Eloquence.
Nahj al-Balagha is an encyclopedia of Islamic culture covering: theology, world of angels, creation of universe, nature of human being, people of different times, good governments and oppressing ones. But the most important point is that Imam (a) did not want to teach natural sciences, zoology or explaining philosophical or historical points. His intention behind mentioning such issues was the same as the Qur'an which preaches using any tangible or rational concept which is understandable for their audiences and take them to the destination they need to arrive at, before God Almighty.
This book has been translated into different languages and many Shi'a and Sunni scholars have written commentaries on it.