The term People of the Book (Ahl Al-Kitāb) is used to refer to followers of certain monotheistic faiths (which have a revealed scripture called Al-Kitab in Arabic which pre-date the advent of Islam. In particular, it refers to the Christians, Jews and Sabians.
“Verily, those who believe and those who are Jews and Christians, and Sabians, whoever believes in Allah and the Last Day and does righteous good deeds shall have their reward with their Lord, on them shall be no fear, nor shall they grieve.” [Al-Baqarah 2:62]
The Sabians of Middle Eastern tradition were a monotheistic Abrahamic religious group. They used to live in Musal (Iraq) and say “LA ILAHA ILL-ALLAH” (none has the right to be worshipped but Allah) and used to read Az-Zabur (The Psalms). They are converts to Islam, while their identity in later Islamic literature became a matter of discussion and investigation.
[The Noble Qur’an by Muhsin Khan]