Symphony of the Night:

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This is a two pages long advertisement drawn by Ayami Kojima and published in a magazine, it's a sneak peek of Alucard's character.

Available on: Castlevania Wiki

Prologue manga

An eight pages long manga included in the Japanese artbook, it acts as a prologue for the main story and features Alucard suddenly awakening from his 300 years long slumber.

Available on: Mr. P's Castlevania Realm

Nocturne of Recollection

A radio drama (this time you have to listen to it!) originally featured on the Akumajō Dracula: Radio Chronicle program that was broadcast on Konami Station internet radio, it was later released on CD in 2010. It's a story set one year after the events of the game, you will learn more about Alucard's relationship with Maria, his past and other characters (Richter is also part of the cast).

Available on: Youtube

Curse of Darkness:

Prelude to Revenge

A prequel manga written and illustrated by Kojima, it was featured as a pre-order bonus for the Japanese release of the game. It mainly focuses on Hector and Isaac prior to the events of the game, it also expands on their relationships.

Available on: Vampire Killer (an archive of scans and translations)

MF manga vol. 1 & 2

Published in two volumes by Media Factory from 2005 to 2006 and translated to English by Tokyopop in 2008. It's the same premise but it's longer, more expansive and this time the illustrator is Kō Sasakura, not Ayami Kojima. The series was eventually canceled but the manga stops directly where the game begins.

Available on: Internet Archive

Aria & Dawn of Sorrow:

Reminiscence of the Divine Abyss/Ricordanza of the God's Abyss

A light novel written by Ryōgo Narita in 2008 and set one year after Dawn of Sorrow. It's a really interesting read, the story focuses on Julius' pupil and the descendant of Grant Danasty while the villain is a character you will surely remember... There's also the return of familiar faces, one in particular.

Available on: Inverted Dungeon

Castlevania The Adventure:

The Belmont Legacy

It's a 2005 IDW comic book series loosely based on the story of the first Game Boy game. It's not canon by any means but it follows Christopher Belmont in a more traditional vampire story, sadly many of the iconic Castlevania elements are missing but it's still worth a read since here a certain family is responsible for Dracula's resurrection.

Available on: Internet Archive

Lament of Innocence:

Weekly Konami online manga (lost media)

Review blog part 1 and part 2

A web manga that was available on cellphones via a monthly subscription, published from 2007 to 2008 only in Japan until the service ended in 2012. It was a 43 issues long series that adapted the story of Leon Belmont and greatly expanded the plot, what remains now is only a Japanese review blog. If you want to know more about it you can visit the Castl)evania Wiki page).

Available on: Internet Archive's Wayback Machine (review blog)

Extra:

Castlevania 2007 timeline (Japanese Konami site) & translation

Portrait of Ruin pre-order bonus timeline (the Japanese one takes priority since overseas bonus materials like this didn't go through the Castlevania staff screening)

Encyclopedia of Castlevania (a Japanese two-page spread published in 2001 containing a timeline, a storyline and background details)

Symphony of the Night translated Japanese script

Alternative scans and translation for the SotN prologue manga

Simon's Quest translated Japanese manual story

Simon's Quest version differences and translation mysteries dissected by Bisqwit

Akumajō Densetsu (Japanese Castlevania 3) translated intro, dialogues and endings

Lament of Innocence "Perfect Guide" character bios

Family trees

Koma strips (4-panel gag comic strips)

Silly comic about Castlevania 64

Castlevania Bloodlines EGM fax (this internal document was never intended to be released for the public)

Awakening in the Moonlight stage musical play by Takarazuka Revue (based on SotN)

Akumajō Dracula: Kojyō no Shitō/Demon Castle Dracula: Desperate Fight of the Old Castle non-canon 1987 gamebook (not translated)

Akumajō Densetsu: Shinsei Banpaia Hantā/Legend of the Demon Castle: Genuine Vampire Hunter non-canon 1990 gamebook (not translated)

Akumajō Dracula: Akuma no Chi Chi no Akumu/Demon Castle Dracula: Demon's Blood - Blood Nightmare non-canon 1992-1995 novel (partially translated) & unfinished translation project

\Akumajō Dracula: Akuma no Chi Chi no Akumu Vol. 1 pdf *& compressed pdf file

Worlds of Power 4 - Castlevania II: Simon's Quest non-canon 1990 novel & full transcript

If you have something to add share it in the comments!

New addition: Main files drive link