The Books of Amulan, Nulama, and Lauman

These books, with their confusing names that are each an anagram of each other, are unique artifacts. They are standalone books that can be rearranged and reordered to create up to 5 individual books on their own. However, if put together and arranged in very specific orders, the books can formulate a Gate spell and even pull specific entities through the gate. This is among other potential outcomes. 
The books as they stand alone are as follows:

The Book of Amulan:

Named from the sunbird (Amuan), this book contains writings of all that is good and positive in the world. It is scripture denoting the heavens and the divine, providing guidance for defeating evil. Few ever can case upon its pages and avoid weeping from the encounter. Only a person of good alignment can comprehend and gain benefits from reading this book. You can continue to hold the book and gain its benefits for as long as you strive to do good in the world. Failing to do a good act every 10 days or actively doing an evil act will cause the book to crumble and vanish. This book is currently in Tipasa, held by followers of Saat. It has not been read for that risks it being lost again, reappearing in another place on the plane. It is also known as the book of Heaven. 

The Book of Nulama:

Named from the corvid (Nauma), this book contains writings of all that is dark and destructive in the world. Though this text may have once only listed rituals for death, the afterlife, and related knowledge, it now contains evil and corrupted knowledge of undeath and necromancy thanks to the corruption of Valravn. Few can stomach what they read without becoming evil themselves. You can continue to hold the book and gain its benefits for as long as you strive to do evil in the world. Failing to do an evil act every 10 days or actively doing a good act will cause the book to crumble and vanish. The location of this book is currently unknown to most, in the possession of Tellefax Naught. It is also known as the Necronomicon or The Book of the Ether, though it is now corrupted into The Book of Vile Darkness. Some believe it could be cleansed again to be of less evil power. 

The Book of Lauman

This expansive book documents the existence of fiends and the layers of Hell. It is the most thorough and blasphemous tome of demonology in the multiverse. The tome recounts both the oldest and most current profanities of Hell. Demons and Devils have attempted to censor the text, and while sections have been ripped from the book's spine, the general chapters remain, ever revealing demonic secrets. The book holds more than blasphemies. Caged behind lines of script roils a secret piece of hell itself, which keeps the book up-to-date, no matter how many pages are removed, and it longs to be more than mere reference material. It is also known as the book of Hell. 

Theorized Additional Texts

Some theories whisper of the existence of other books. Though only three of any or all would be needed to create a Gate spell, some say these would be essential to commanding the power of specific otherworldly entities. Those books are theorized to be

The Book of Fairie: a comprehensive knowledge to help one better understand the laws of the fey, the otherrealm, and all that exists within. 
The Morellonomicon: a comprehensive text and the spellbook of a once-great wizard named Morello. It details the power of the weave, lay lines, and the powers of mortality. It exists almost as a text of the powers of the material plane and the weave that exists within.