Here you can post your attempt to list the books called "Kitaab at-Tawḥīd" written by the scholars of Islām throughout history. Six were mentioned in the introduction to our course, and perhaps some of you may know of some additional books by the same name.
The six which were mentioned in the course introduction are as follows:
(1) Kitaab at-Tawḥīd in al-Jaami as-Ṣaḥīh by Imām al-Bukhārī (d.256AH).
(2) Kitaab at-Tawḥīd by Ibn Khuzaymah (d.311AH).
(3) Kitaab at-Tawḥīd by Ibn Mandah (d.395AH).
(4) Kitaab at-Tawḥīd by Ibn Rajab (d.795AH).
(5) Kitaab at-Tawḥīd by Muhammad Ibn Abdul-Wahhab (d.1206AH)
(6) Kitaab at-Tawḥīd by Ṣāliḥ al-Fawzān - may Allaah preserve him.
1) Kitaabut-Tawḥīd by al-Bukhari (in his Ṣaḥīh) [b. 194, d. 256H.]
2) Kitaabut-Tawḥīd by Abū Bakr Muhammad ibn Ishaaq Ibn Khuzaymah [b. 223, d. 311]
3) Kitaabut-Tawḥīd by Abū 'Abdillaah Muhammad ibn Ishaaq Ibn Mandah [b. 310 or 311, d. 395 or 396]
4) Kitaabut-Tawḥīd by Ibn Rajab al-Hanbali [b. 736, d.795]
5) Kitaabut-Tawḥīd by Muhammad ibn 'Abdul-Wahhaab [b. 1115, d. 1206] (may Allah have mercy upon them all)
6) Kitaabut-Tawḥīd by Shaykh Ṣāliḥ ibn Fawzān al-Fawzān (may Allah preserve him)
Kitaab at-Tawḥīd: Abū Abdillah Muhammad bin Ishaaq bin Muhammad ibn Mandah[d. 395]
Kitaab at-Tawḥīd: al-Bukhārī [d. 256] (derived from his ṣaḥīḥ)
Kitaab at-Tawḥīd: ibn Khuzaymah [d. 311]
Kitaab at-Tawḥīd: Ibn Rajab 
Kitaab at-Tawḥīd: Ibn Abdil Wahhaab [d. 1206]
Kitaab at-Tawḥīd: Shaykh Ṣāliḥ al-Fawzān
The six books called "Kitaab at-Tawḥīd" written by the scholars of Islām, which were identified in the introductory course are as follows:
1)Kitaab at-Tawḥīd in al-Jaami as-Ṣaḥīh by Imam al-Bukhari [d.256AH]
2)Kitaab at-Tawḥīd by Ibn Khuzaymah [d.311A]
3)Kitaab at-Tawḥīd by Ibn Mandah [d.395AH]
4)Kitaab at-Tawḥīd by Ibn Rajab al-Hanbali [d.795AH]
5)Kitaab at-Tawḥīd by Muhammad Ibn Abdul-Wahhab [d.1206AH]
6)Kitaab at-Tawḥīd by Shaykh Saaleh al-Fawzān - may Allaah preserve him, ameen.
Regarding #7, I found it on the net and that's the title on the front page (i.e.: Kitaabut-Tawḥīd liLlaah 'azza wa jall); but going through the book a bit it mentions the title just as: "at-Tawḥīd liLlaahi 'azza wa jall" - so, I'm not sure if it still counts.
It was verified & its aḥadīth referenced by: "مصعب بن عطا الله الحايك," published 1419H (1998) by Darul-Muslim, Riyaadh.