[teja208]

Originally Posted by houkoholic

If you piece together the clues it seems fairly obvious:

Spoiler for clues and speculations:

Dekomori in this episode said she got this (massively chuuni) book two years ago from the "magician" named Mori Summer (as pointed out Mori/forest=森　Summer/natsu= 夏 hence combine the two kanji you get 森夏 = Shinka).

And from the chuuni character description above taken from the official site Shinka is "fighting Rikka's servant (ie Dekomori)" and "looking for the past legacy" so if Mori Summer = Shinka then this implies she's going to try to get her book back from Dekomori.

From episode 2 Shinka also said she's just pretending she is dependable and all that etc so the obvious conclusion is that Shinka is like Yuuta who wants to bury her chuuni past and hide the fact that she used be a chuunibou and begin to live a new normal life when entering high school, and because that book is the legacy of her chuunibou she can't let someone know how bad she was and wants to take it back, thus the way to do it is to join the club so she has more chances to get close to Dekomori.

Spoiler for More speculations:

In addition to what you speculated, remember that Shinka also said that she and Yuuta are alike, and the chuuni description also says she opposed the Devil Eye i.e. Rikka which could mean that she's trying to persuade Yuuta, as ex-chuunibyou like her, should distance himself from Rikka. I personally hope I'm wrong about this since it would make Shinka a bitch in sheep's clothing.