I just finished watching Death Note, but I am still confused whether I watched the full story or not. I binged through the anime with 37 episodes.
For example, there is Beyond Birthday's theme on YouTube, but I never heard his name in the anime?
What is Death Note Relight? What is Death Note Rewrite? Where can I get these?
How many books/novels about Death Note are there in total?
The core of the Death Note series is comprised of the 108 manga chapters created by Tsugumi Ohba and Takeshi Obata and its 37 episode anime adaptation which covers the same material. Consuming either covers the entire main story of Death Note and everything beyond that is supplemental which you can peruse if you want more from the Death Note universe.
In roughly chronological order the pieces of Death Note supplemental media are:
The Death Note one-shot (2003) is the pilot chapter for the Death Note manga which does not share a continuity with the main series and features a different main character and a Death item not seen in the main series.
Death Note 13: How to Read (2006) is the unofficial 13th volume of the manga which is an encyclopedia style tie-in book for the Death Note manga featuring character profiles, interviews with the creators, etc. along with an exclusive set of yonkoma (4-panel) comics.
Death Note Another Note: The Los Angeles BB Murder Cases (2006) is a prequel tie-in light novel penned by Nisio Isin (not the duo who made the Death Note manga). The story features Mello's retelling of a case L was involved in prior to the start of the main series.
The live action film Death Note (2006) is a mostly faithful adaptation of the first 2 volumes of the manga and its sequel Death Note 2: The Last Name (2006) continues the story where the previous film leaves off but veers away from the source material and concludes with a film original ending which differs from the manga/anime.
These live-action films lead into the spin-off light novel L: change the World (2007) penned by someone going by the pseudonym M and its film L: change the World (2008) which are both spin-offs from Death Note 2: The Last Name and not the Death Note manga due to the differences in endings.
The anime films Death Note: Relight - Visions of a God (2007) and its sequel Death Note: Relight 2 - L's successors (2008) are cinematic recaps of the first and second half of the main story respectively with a framing story involving an unnamed Shinigami asking Ryuk for a tale and Ryuk retelling the events of Death Note.
The live action film Death Note: Light Up the New World (2016) and its associated light novel adaptation Death Note: Light Up the NEW World Film Novelization (2016) are a direct sequel to Death Note 2: The Last Name and not the manga. This is a wholly original story set 10 years after the events of DN2: TLN.
The C-Kira story (2008) and The A-Kira story (2020) are epilogue one-shot manga made by the original creators which take place after the events of the main series detailing the further exploits of Ryuk. These one-shots along with the pilot chapter, a pair of short manga chapters (L-One Day and L-The Wammy's House) and the yonkoma from Death Note 13: How to Read were collected into a collection titled Death Note: Short Stories (2021) which is set to be released in the West in 2022
We're going to stop here because this already covers a huge breadth of supplemental material beyond Death Note's core story and the rabbit hole goes even deeper to include things like a photo book, video games, a musical and the Netflix adaptation featuring everybody's favourite American high-schooler Light Turner.
