
All these books you have listed touch on the core of Aqeedah (Islamic Belief) and thats Tawheed. Usul Thalaatha and Qawa'idul Arba'a along with Kitaab Tawheed are all written by Shaykh Muhammad bin Abdul-Wahhab and they were written in a time when the arabic peninsula was engulfed in minor aswell as major Shirk and all these books the Shaykh wrote deals with the sound Aqeedah every muslim should have and it warns against Shirk in its many different forms.
As for Aqeedat-ul-Wasatiyah, its written by Shaykh-ul-Islaam Ibn Taymiyyah.
Usul-Thalaatha
Shaykh Muhammad ibn 'Abdul-Wahhaab, rahimatullah, who wrote this in order to convey to every Muslim that which is obligatory upon him to know - in fact, the "three principles" are based on the three questions that we will all be asked in our graves.
Qaw'idul Arba'a
In this book, the author explains the dangerous concept of shirk through four simple principles. The sin of Shirk- or associating partners with Allah - is the most reprehensible act that a person can do. It is for this one reason that that this sin is the one sin that Allah has promised He will never forgive. Unfortunately, many muslims who are unaware of the reality of shirk fall into it out of ignorance. This short treatise is an ideal work for the muslims in order to understand the essence of shirk, and thus essential reading for the one that wishes to avoid falling into the greatest trap of Shaytaan.
Kitaab Tawheed
The book, as the title suggests, deals with the subject of worshipping Allah alone without associating partners with Him. The author highlights many matters that have become common among Muslims even though they are clearly acts of shirk or associating partners with Allah. Not only is this book distinguished due to the importance of the subject matter but also due to the methodology adopted by the author for which he is well known. The reader will find that the majority of the book consists of verses of the Qur'an, ahadith of the Prophet (pbuh), and sayings of the early Muslims.
Aqeedat-ul-Wasatiyah
al-Aqeedah al-Waasittiyah, one of his (Ibn Taymiyyah) more famous books, was written in response to a request from one judge from Wasith. He asked ibn Taymiyyah to write his views about theology in Islam. This book contains several chapters. In the first chapter ibn Taymiyyah defines one group that's called al-Firq an-Najiyah (the group of survival). He quoted one Hadith in which Muhammad promised that there will be one group of his followers to stay on the truth until the day of resurrection. This chapter also contains the definition of jamaah and states that only one of the seventy three Muslim sects will enter Jannah.
Chapter two contains the view of Ahl as-Sunnah wal-Jamm'ah regarding the attributes of Allah based on the Qur'an and Sunnah without ta'teel (rejection), tamtsil (anthropomorphism), tahreef (changing Allah's attributes), and takyif (questioning Allah's attributes). This book also contains the Six articles of belief for Muslims.