The Codex is basically an encyclopedia about the Shadowhunter world. It contains a lot of information about things like Shadowhunter culture, Downworlders, demons, magic, weapons and combat, Idris and the history of Shadowhunters.
But this isn't just any copy of the Codex. It is actually Clary's book and she, Jace and Simon have written all kinds of commentary in the book.
This series provides examples of:
- Brother–Sister Team: The very first Shadowhunters included a pair of siblings: Jonathan Shadowhunter and his older sister, Abigail. Abigail later became the first Iron Sister.
- Deliberate Values Dissonance: The book details things that weren't as rosier for Downworlders in the past, when they were actively persecuted by the Clave and weren't allowed to marry Shadowhunters in absence of the Accords.
- Fantastic Racism: The codex is a little racist in its chapters about Downworlders. It also regards mundanes in a patronizing way. Since it's written by the Clave, this should come as no surprise.
- Get a Room!: Simon writes this in the book when Clary and Jace's comments started to get all lovey-dovey.
- Hidden Elf Village: The history section tells that Shadowhunters weren't always elitist towards mundanes as they are today. In the past, Shadowhunter-mundane marriages were perfectly legal and mundanes didn't have to undergo Ascension to marry Shadowhunters. Around the Protestant Reformation, however, things changed as the Clave increasingly isolated themselves, to the point that merely associating with mundanes was considered forbidden. However, the Clave couldn't really control the vast expansion of mundane activities that accelerated since the modern period, so relations with mundanes were legalized again, but mundanes have to undergo Ascension if they want to marry Shadowhunters.
- Lemony Narrator: Clary, Jace, and Simon wrote scribbles throughout the Codex, annotating and criticizing parts that are of interest in their experience. There is also a "Shadowhunter Rules" section written by Magnus, which mostly consists of him ribbing on the Shadowhunters' tendency towards arrogance and superiority over Downworlders.
- Really 700 Years Old: The oldest living warlock was born in the 7th century. The second oldest, the older sister of Elphas the Unsteady, lives in Alicante, though she is now a doddering old lady.
- Unreliable Narrator: The book is written by the Clave, so it is pretty biased in favor of the Clave and Shadowhunters in general. For example the book states that Praetor Lupus was founded as a form of self policing by werewolves to protect others from the dangers new werewolves pose, while in reality they were founded to prevent the Clave from killing new werewolves.
- Unintentional Period Piece: In-universe. On the section detailing the Mortal Instruments, the Codex states that the Mortal Mirror is lost in history. Since she knew the truth about just what it is, Clary wrote "Seems obvious once you know the secret, huh?".
- The 'Verse: Part of The Shadowhunter Chronicles consisting of this book, The Mortal Instruments, prequel series The Infernal Devices, sequel series The Dark Artifices, The Last Hours (sequel to The Infernal Devices), The Wicked Powers (sequel to The Dark Artifices), The Eldest Curses (spin-off focusing on Alec and Magnus), The Bane Chronicles (short story collection), Tales from the Shadowhunter Academy and Ghosts Of The Shadowmarket (short story collection), all taking place in the Shadowhunter world.
- We Used to Be Friends: The warlocks used to be steadfast allies of the Shadowhunters; Jonathan Shadowhunter was friends with Elphas the Unsteady, who helped him write down accurate texts on the supernatural. Then the Schism happened, during which warlocks were rounded up and executed by Shadowhunters in the dozens. Since the Accords, things have been improving, but the relationship hasn't exactly returned to the times of Jonathan and Elphas.
- Witch Hunt: During the mundane witch hunts of the early modern period, Shadowhunters helped mundanes round up heretics, who in their case consisted of werewolves, vampires, and the occasional warlocks.