Assalaamu alaykum. How authentic is Nahj Al-Balaaghah? It is the book in which sayings of ‘Ali, may Allah be pleased with him, are mentioned and which is mostly quoted by Shias.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

If by the authenticity of the book you are inquiring about the authenticity of its content, then it includes tremendous falsehood. If you are asking whether or not it is attributed to ‘Ali then it is not valid to attribute it to him.

Ibn Taymiyyah said in Minhaaj As-Sunnah:

"These speeches that are quoted in the book (Nahj Al-Balaaghah), if they were all truly said by ‘Ali, then they would have been quoted from him before this book was written with and without their chains of narrators. If scholars with experience in narrated quotations know that many of them, or rather most of them, were not known before this book, then it becomes clear that this is a lie; otherwise, the one who quotes them has to mention from which book they were quoted or excerpted and who quoted them from ‘Ali and what their Isnaad (chain of narrators) is. If not, then anyone can make unfounded claims."

Ibn Hajar said in his book Lisaan al-Meezaan, "The one who reads Nahj Al-Balaaghah would assert that it is a lie against the Commander of the Believers, ‘Ali because it includes clear insults and degrading of Abu Bakr and ‘Umar . Also, the book includes many contradictions and weak expressions and phrases, and the one who is familiar with the nature of Qurayshi Companions and others who came after them would definitely assert that most of the book is falsehood.'"

Allah knows best.