@Background Pony #494E  
I’m sure that Sunset, Sonata, and magic music fights can carry the whole thing.

So far what we see, this movie got Plot convenience, Deus ex machina, and some plot twists. Also, Predictable story

@Background Pony #36B4  
There is another shot around, within the last week, which is from the spanish VA taking some teasing photos.
 
One shot shows the mane 6 (ponies) around a table with their version of the book (looking like Celestia’s CM on it) and having a late-film time code.
 
My guess is that yes, between the library being able to power the portal, and the book being a persistent thing, they have a way to keep any EQG concepts going. I am absolutely not expecting any human-crossing-to-pony on the show within S5 but they have a way to launch a third movie and/or that Canterlot Academy thing that’s been floating around.

@Monfang  
Book’s pretty dusty, dawg.
 
@OmegaCrowley  
Magic book that sends messages to its counterpart in Equestria. It’s how Sunset sends Twilight Sparkle an SOS.

@Masem  
and hopefully harmony magic energy can create a more or less stable portal in comparison.

@PonyPon  
……… i should hate this but i find it far too fucking hilarious.

@Monfang  
It looks like the crystal treebrary is helping to power the mirror when she uses it this time, which no one know about in the past movie
 
Also to take Reflections as canon, maybe SSTB popped that spell on that version of the mirror to only open limited times as to prevent the same thing that happened with the Sombra-verse, and Twilight’s magic + the crystal treebrary are able to override it

@Monfang
 
Oh believe me, I’m rolling my eyes at that too. Although it does prove that Twi is a scientific/magical badass. That’s the only positive aspect, I guess? :P

@Tchernobog  
faceplams all around!
 
you guys are not taking into account how this narratively fits into Celestia. we know she forms a strong bond with her personal apprentices, almsot a parental one. and had a means of communication with twilight too. seems to me that celestia would have a personal means of communication with sunset shimmer.
 
for all her bluster, her keeping this book might show her insecutirties and doubts over her actions. she could not bring herself to dispose of the book despite not longer being celestia’s apprentice. likely some good thoughts and memories are connected to that book.

@Masem  
Dear Sunset, I hope you’re finding whatever it is you’re looking for over there.
 
I know I have.  

 
Missing you always, Princess Celestia.
 
………….
 
 
Dear Princess Celestia,
 
(what looks like a massive spit-stain on the page)

@Tchernobog  
As big as Twilight making a machine that breaks the ‘x moons required for visits’ rule?
 
Honestly, that is the worst part for me. What was wrong with getting THAT world’s Twilight to do the magicy things?

because she still cared about celestia i presume. while too subtle ot be caught on so easily, it think a large part of sunset’s behavior last movie was jealousy over tiwlight. celestia behaves much like a parent, and tiwlight is her more successful, younger sister in that sense.
 
more than that it seems to sow tht she had more doubts about her actions than she let on if she kept it.

HOLY PLOT CONVENIENCE BATMAN!
 
This book… is a rather big ass-pull for the plot. :/

@Monfang  
Now that is an interesting idea, if true.

It answers how she knows what was going on in Equestria while in exile.
 
She has an inside mare.

Sunset’s family history? That is her cutie mark

My take is that while she was sure of leaving Celestia and that all behind, this was a possible lifeline if it turned out she was wrong and needed her help. But as she didn’t need it for the three-ish years (implied) while on the human side, she just let it slide.

You’d think she would.
 
I would think she would.
 
But I wonder more how it would even function properly.
 
But that shows what I know.