Much of this book is plodding and confusing. The author introduces a lot of new lore and spells without properly explaining them, and then keeps referencing them as an integral part of the plot.
The characterization is also poor. The crux of the story is that Starlight and Twilight are struggling with communication. Their issues go somewhat like this:
Twilight: Hey Starlight, let's study this new magic spell I'm working on.
Starlight: We've been working too hard Twilight, I'm tired and want to take a break. :)
Twilight: Okay. :)
~Later~
Starlight: I need to use this secret room I found to hide from Twilight, even though she understood how I was feeling and I wasn't mad at her!
Starlight: *practices spell by herself, messes up time in Ponyville, doesn't realize it was her own fault and lets Twilight take the fall*
~Later~
Twilight: Hey Starlight, I'm sending you to go spend time with your friend Sunburst so you aren't so stressed out!
Starlight: Okay!
Starlight: *proceeds to stock up on a bunch of food and hide in the secret room instead, practice magic by herself again, and create a huge swirling vortex that makes everything happen in slow motion*
~While vortex is swirling in room and threatening Ponvyille~
Twilight: I was a bad communicator. :(
Starlight: Me too. I forgive you. :)
Twilight: :)
Spike: MAYBE FIX THE VORTEX AND TALK ABOUT THIS LATER?????
So yeah. This was pretty bad. The conflict isn't well-handled, the pacing is messy, the spells are confusing, and the characters are mostly lame. The only thing I DID like is that Starlight is further developed as an introvert who needs time by herself to recharge, and that she isn't a workaholic like Twilight is. I actually liked the idea of this book, but for some reason the author lost her usual charm and made Starlight a typical children's book protagonist who never feels anger (or anything but love and compassion) towards her friends. So instead, she came across as unrelatable and incredibly stupid.