This is for those interested in the original story from which this drama was adapted and for ARMYs familiar with BU universe who want to refresh their memories. I myself forgot a lot of details, especially from HYYH notes. 

1. Original BTS MVs and short films that are part of BU/HYYH storyline (YT playlist)

2. HYYH Notes (this is from album booklets and published books - The Notes 1 & 2)

3. Save Me webtoon

You can find more additional information on this site: https://www.usbtsarmy.com/bangtan-universe-guide. It's super organized and easy to navigate.