Is the word "الصابئون" (al-Sābi’ūn) in verse 69 of Surat Al-Ma’idah considered a grammatical error, and why did "الصابئين" (al-Sābi’īn) appear in Surat Al-Hajj, verse 17? Are there books that refute these misconceptions, and what are they?

The claim of linguistic errors in the Quran is refuted by its very nature. Allah challenged the Quraysh and the Arabs who resisted Islam to produce a chapter like it, and none of them ever made such a claim. Instead, they said, "This is naught but an invented magic." Scholars have clarified the verse that some found problematic, such as the verse: "Indeed, those who believed and those who were Jews and the Sabeans and the Christians," and they mentioned several grammatical explanations for it: 1. Fronting and Postponing (Taqdim wa Ta'khir): The Sabeans and the Christians likewise [are included]. They provided poetic examples to illustrate this. 2. "Inna" meaning "Na'am" (yes/indeed): In this case, "al-Sabi'un" (the Sabeans) would be a subject (mubtada'), and its predicate (khabar) is omitted because the second part of the verse indicates it.

From the above, the injustice and flimsiness of these misconceptions become clear. For more details, one can refer to books of Tafsir (Quranic exegesis) and books on the grammar of the Quran.