A liber spirituum sometimes referred to as a book of spirits is a book much like a book of shadows, but instead of containing spells, rituals, and other occult knowledge these books contain information on certain entity, so you can summon them. The liber spirituum is a book in which a practitioner, or a group record a collection of entities they work with, or summon. This information is written down in this book, so that a practitioner could call on it again if he needed its assistance, it was a lot like a hand-written phone book for certain entities. These books are traditionally made by the practitioner, and are also traditionally handwritten.
This will act as a practitioner’s own personal list of spirit names. As the practitioner encounters, and gets the names of more entities they will be able to add them to their liber spirituum, so that they can call on them later. These list do not have to be complicated, and all they really need to include is the spirit’s name, so from that a skill conjure can summon the entity. These books also usually tell the conjurer a lot more about certain entities in the book, so they know what they are getting into before they summon them. These books should be handled with the utmost care, because of the information that it contains. This book will be very private, and personal to the practitioner, or group that watches over the information, and to the entities that the information is about. These books are even sometimes blessed and/or consecrated by the end of that dwell within it’s pages.
if you are interacting with many different entities you could be writing down their information, and you could create your own liber spirituum, so that you could keep track of all the entities you meet and summon them later, if needed. The information that could be provided on the entity will be such things as the name, sigil, personality, description, methods on how to summon, many different associations, and correspondences, likes, and dislikes, and maybe even an image of the entities appearance. It is very important to a record as much information as you can on certain entities, so that you can remember important details about them. The more information you get usually the better, you never know what information will be important in the future. The more you interact with different entities, and the more you learn about them, the more you will be able to write in your liber spirituum.