Your patron is only known as The Athenaeum, Ruled by Pep the god of the library. The Athenaeum is a being of nearly endless knowledge and curiosity. It was formed by the collective knowledge of the world bound in shelves and bindings, and by the residual magic of the most powerful wizards and sages, who recorded their arcane findings on pages and in tomes. The Athenaeum is neither outwardly benevolent nor malevolent, being very orderly and neutral in its actions. The Athenaeum does not seek out creatures to complete its goals, rather, it tests those that have promise in securing dangerous artifacts and maintaining order. The warlocks that make The Athenaeum their patron are known as Librarians, and are often scholars or sages in practice. Dungeon Masters who have a player with the Library Patron are encouraged to have their warlocks seek and secure powerful artifacts as well as keep the peace and silence in the library, and any Librarian who abuses said artifacts should be punished accordingly.

Expanded Spell List

Athenaeum Expanded Spells

Intellectual Spellcasting

The nature of your patron changes the very nature of being a warlock. Your spellcasting ability is changed from Charisma to Intelligence, so you use your Intelligence whenever a spell refers to your spellcasting ability. In addition, you use your Intelligence modifier when setting the saving throw DC for a warlock spell you cast and when making an attack roll with one, and all warlock features that use your Charisma score/modifier for the feature instead of your Intelligence sore/modifier.

Spell save DC = 8 + your proficiency bonus + your Intelligence modifier

Spell attack modifier = your proficiency bonus + your Intelligence modifier

You also gain the ability to cast warlock spells as rituals:

You can cast a warlock spell as a ritual if that spell has the ritual tag.

Library Book

At 1st level, The Library has gifted you a book to take on your journeys. Through this book, you maintain a connection to your patron, as well as the knowledge it contains. The book will automatically record information that it detects around it and gives you abilities while it is in your possession. You are also given permission to enter The Library

• You are bonded to your book. You can summon it to yourself as a bonus action if it is on the same plane of existence, causing it to teleport instantly to your hand.
• Your Library Book can serve as a spellcasting focus, for the purpose of casting your warlock spells.
• You can read your book to accurately recall anything you have seen or heard while you have had it in your possession.
• You can perceive through the book, reading it in your mind's eye as it transcribes what is around it. You can use this ability regardless of the book's location. Darkness does not affect the perception of the book. 
• While holding the book open in your hand, you gain proficiency in Arcana, History, Nature, or Religion. 

When you gain your Pact Boon at 3rd level, if you take the Pact of the Tome, your Library Book doubles as your Book of Shadows, gaining triangular gold plates on the corners of the binding of your book. If you lose your Book of Shadows, you may use either form of regaining it, provided by the Pact Boon or by the Library Book feature.

Body of Knowledge

At 6th level, as an action, while holding your book, your body merges with it. While merged;

• You become Tiny and appear as a simple book, although anyone that detects magic will see it as Divination.
• Your speed is reduced to 0
• You automatically succeed in any check to hide, assuming that the creature didn't see you merge and has no reason to suspect the book.
• You can only communicate telepathically with any creature you can see within 30 feet of you. They cannot reply telepathically.
• You can use your action only to remove yourself from the book, cast a touch-range spell, or perform a Paper Cut attack, dealing 1d4 slashing damage to anything holding you. 
• When you choose to leave the book, you can decide to instead come out of the pages of any other book within 60 feet of you. 

You can use this feature a number of times equal to your Intelligence modifier (minimum of 1) plus your proficiency bonus. You regain any expended uses when you finish a short or long rest.

While taking a long rest, you can use this feature to visit the Library in your dreams, allowing you to research. You are unable to focus on the outside world unless you are shaken from your research, and if you are shaken from your research you are only able to perceive anything within 30 feet of the book.

Knowledge is Power

At 10th level, you gain the ability to take one other willing or non-willing creature within 5ft. with you when you merge with your book. You both perceive the world as it happens in real-time through a flow of text. If the creature is not hostile, it gains all the abilities that you do when merged with your book, with the added ability that you can communicate clearly with each other. If the creature is hostile The must pass a Dc 15 Wisdom save or be merged with the book, both the creature and your attacks do psychic damage and can only attack each other. The book bounces comically on the floor while a fight is occurring in its pages. If you or another creature dies while in the book, both of you are expelled from its pages, unconscious. You can use this feature a number of times equal to half of your Intelligence modifier (a minimum of once). You regain any expended uses when you finish a long rest.

Knowledge is Eternal

At 14th level, you have built up a powerful connection to your patron through your book. 

• Any creature you have defeated is recorded in the book and you can recall the information about them at will.
• If you are reduced to 0 hit-points, your book absorbs you to save you from death. If you fail your death saving throws within your book, you remain inside the book until you can be revived, or until you are returned to The Library. While inside your book, you are incapacitated, but not unconscious. Should your book be destroyed while you are inside it and have 0 hit-points, you are ejected from your book and you die.
• As an action, as long as you hold your book can create a gate to any of the locations recorded in your book. This can be done up to your proficiency bonus and resets after a long rest.
• As an action, you may enter and exit through any book within a 300ft radius. This can be done up to your proficiency bonus and resets after a long rest.