Theories have it that the record Key puts on is a small reference to Jjong.
It's a Michael Jackson album that Jjong had on display on his mantle.

And the book Minho is reading.
The book being "The Ruby Knight" by David Eddings.
In this theory the book is part of a 3 book series and has to do with 3 different colors.
So was The Story of Light. 3 parts, 3 colors.

The first book is called "The Diamond Throne" and diamonds, obvi.
Anyway it's about 5 knights on a quest to defeat evil basically. Obvi again. But also crying.

The 2nd book is "The Ruby Knight" and it's about hope.
This is the book seen in the MV. Also the flowers that Taemin is holding at the end are supposed
to symbolize (a sapphire in the book) said hope. And love.

The 3rd and last book "The Sapphire Rose" = Healing.
But before there can be healing, there is a great loss and with this loss the knights have changed.
 

But, that is the short abridged version.
OP then explains each Story of Light MV.
You can read the full thing here (x)