While each magician has their own spellbook, there are some spellbooks that are so well-known that they are named and have been copied through the various kingdoms of the world. A mage might find one of these spellbooks or an excerpt of one during their travels.
The Black Sea Scrolls* – These books are necromantics in origin and are said to be the source of the original vampiric curse.
The Book of Oshtur – Said to be penned by the Bright Lady herself, this book has spells that aid any spellcaster in defending themselves from the attacks of all manner of creatures but most particularly from extraplanar threats.
The Keys of Doom – This set of spellbooks were the books of Baron Doom (later King Doom) and contain a wide variety of spells, particularly within the evocation school.
De Nigromancia* – A treatis on black magic as revealed to its author by way of conversations with a wraith.
The Magus aka Celestial Intelligencer – A complete system of occult philosophy, a generalized spell book commonly given to neophyte mages.
Le Dragon Noir* – A book of necromancy focusing on death magic
Le Dragon Rouge* – A book on demonology and binding the fires of Hel to this world
Le Dragon Blanche – A book on divination including a treatis on Hevenly beings
Le Dragon Verte – This grimoire describes how a mage can use their own life energy or that of others to defend themselves focusing primarily on abjuration magics
Le Dragon Jaune* – A book on necromancy focusing on disease
Le Dragon Blueus – Written by a trio of authors this book discusses ways to influence others both subtly and overtly and details many enchantement magics
Le Dragon Gris* – A book on necromancy focusing on undead
Grimoire of Stephen – A catalog of extraplanar threats with instructions for sealing them from entry to our world. While primarily a treatis on abjurational magics it does contain some other spells related to combating extraplanar threats.
Grimoire of Mordo* – A grimoire containing seals and sigils of important demons and spells for binding said creatures as well as supporting spells.
Grimoirium Verum – On the surface this work appears to be a general treatis on magic as a theoretical endeavor, but on a closer read it contains numerous illusion magics.
Spirits of Power and Truth* – This work contains an eclectic mix of spells but also has numerous formula for the creation of necromantic items of power.
Wonderful Secrets of Natural Magic – This work contains transmutation magics as well as forumla for the creation of some of the more common items of power used by wizards.
*These spellbooks contain information that is considered Necromancy or Demonology related and are probably restricted in most kingdoms and possession of said spellbooks is likely to get the mage in trouble with the local authorities if not a local mob.