What is this book?
When not reading the Abyssalnomicon
This is the Necronomicon, a book written by Abdul Alhazred, also known as "The Mad Arab". The book contains knowledge about The Great Old Ones, their names and how to summon them. There is also information about spells and rituals the book can perform.
The Necronomicon itself is a chapter of the Abyssalnomicon, which was read to Alhzared by a demon known as J'zahar, who had composed the Abyssalnomicon from infinite knowledge know to The Great Old Ones.
There are 4 types of necronomicons within the Abyssalnomicon: The Necronomicon, The Abyssal Wasteland Necronomicon, The Dreadlands Necronomicon, The Omothol Necronomicon, each with knowledge bound to a specific dimension.
In order to unlock the knowledge, one must travel to said dimension and empower the Necronomicon with the flesh of that world. The Abyssalnomicon contains additional knowledge known only to J'zahar, knowledge man was not meant to obtain.
When reading the Abyssalnomicon
This is the Abyssalnomicon, a book written by J'zahar, Gatekeeper of The Abyss. The book contains the knowledge of the 4 Necronomicons, as they are in fact chapters of this book. There is also extensive knowledge not mentioned in the others.
As the knowledge about spells and rituals is limited to the specified dimension (and previous ones, depending on the Necronomicon type), the Abyssalnomicon has all knowledge collected in one place, with additions.