Giving the fact that there are more than 4 Holy Books (Psalms, Torah, Injeel, Quran) but not named; besides Jews and Christians, which nations in the Quran can be considered as the people of the book.
From this answer 'Who are Sabeans?' I understand that Sabians are considered among the people of the book.
Do Madeans count?
The Mandaeans re-appear at the beginning of the Muslim conquest of Mesopotamia, when their "head of the people" Anush son of Danqa appears before Muslim authorities showing them a copy of the Ginza Rabba, the Mandaean holy book, and proclaiming the chief Mandaean prophet to be John the Baptist, who is also mentioned in the Quran. The connection with the Quranic Sabians provided them acknowledgement as People of the Book, a legal minority religion within the Muslim Empire. They appear to have flourished during the early Islamic period, as attested by the voluminous expansion of Mandaic literature and canons. Tib near Wasit is particularly noted as an important scribal centre. Yaqut al-Hamawi describes Tib as a town inhabited by Nabatean (i.e. Aramaic speaking) Sabians who consider themselves to be descendants of Seth son of Adam.
The status of the Mandaeans became an issue for the Abbasid al-Qahir Billah. To avoid further investigation by the authorities, the Mandaeans paid a bribe of 50,000 dinars and were left alone. It appears that the Mandaeans were even exempt from paying the Jizya, otherwise imposed upon non-Muslims.