All of IDOLiSH7's in-game main story and more, translated into English!
Main Story: This is the most important, and you should probably try to work through this before heading into other material based on the game's story. It's fully voiced by everyone's voice actors.
Special / Event Story: Released either with certain events, or with certain real life celebrations. Some are voiced, most not.
Collaboration Story: Stories that come with the collaboration of the game with other series.
Rabbit Chat / TV: Chats are texts with Tsumugi; TV is a behind the scenes look at the idols. You can find these on the card pages themselves.
In terms of Spoiler Level for everything besides the Main Story, it's probably best to finish reading Part 1 of the Main Story so you have an idea of what's going on.
For the purposes of translations, the player character will be referred to by her default name of Tsumugi Takanashi.
You can find all of the CGs found in the stories here! There are spoilers here however.
In Part 2 and onwards, the unlocking story system changed a bit. Now in order to progress the story you need to complete that Story Live requirements which will earn you bookmarks, you only need two bookmarks to unlock the Chapter.
It's not required to get all three bookmarks as you only need two to progress. You don't get anything special for getting all of them. Actually, you don't get anything at all.
A story live costs 30 LP at first, but once you get two bookmarks it will cost 15 LP.