Time to hop on the bandwagon initiated by these fine gentlemen and do one of these. Rather than include cards that see play or are otherwise memorable, I tried to pick eight cards that actually represent spells Starlight has been shown to cast. Plus it gives me an excuse to be lazy and skip the art descriptions and flavortext. You guys can make a game out of figuring out what each card is referencing. They're all pretty easy, except for the one that's mostly just a bad joke. Enjoy!

Bottled Cloister
Crystallization
Domineering Will
Levitation
Sudden Disappearance
Switcheroo
Time Warp
Welcome to the Fold

>no original Mind Control/Control Magic

Signature Spellbook: Glimglam is literally just eight different mind control spells.

Mind Control
Soul Ransom
Act of Treason
Volition Reins
Vessel of Volatility

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Nah, Thought Prism has it right that it needs to include time magic as well.

Maybe Spelltwine, since she's apparently a fan of mixing spells together.

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I mean I was exaggerating for the point of comedy.
Signature Spellbook is all about getting to the core of the essence of the character in question, not necessarily every single thing she's ever been seen to do. There would certainly be a single time-based spell in there, my vote would be Day's Undoing. Welcome to the Fold seems like a brilliant choice, though the spellbook is flat out incomplete without a core of mainstay classics like Mind Control.

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I'm guessing Sudden Disappearance is for all those Season 6 episodes where she never shows up?

Also, I'm tempted to suggest Ixidron, since it renders all other creatures equal.

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Well, I suppose it's my turn to be nitpicky and offer my own clearly superior opinion.

The original Spellbook was Jace's planeswalker card and seven cards for his signature spellbook, not eight. As was pointed out by Sesquame, the selection of spells is supposed to represent the character, not attempt to portray every "ability" they have in the show. As such, you'd have to imagine a Starlight Glimmer planeswalker card, give that planeswalker some loyalty abilities to represent what Glimmy is capable of, then accompany that with seven spells to more or less showcase her style and personality.

Glimmy's style feels like some combination of blue or white and red or green. She has the blue feel of a traditional wizard in that she uses magic, is good at magic, and so on. But she doesn't feel like a traditional pure blue mage in that she isn't especially logical or "brainy." She's emotional and fuels her magic with that emotion, which screams of green or red to me. She isn't particularly "nature" or "fire/earth" though, but anger is clearly well within her range of emotions, and I feel that leans more towards red. Some of the things she does being "controlling or manipulative" could easily fall under white, and at least some of her style is related to the community. Old Glimmy was probably more Red/white, while new Glimmy feels more Blue/red to me.

FOME's suggestion of Ixidron is pretty brilliant if we want to focus on Glimmy pre-redemption. Mycosynth Lattice and Humility also fit this part of her arc. Humility, in particular, I feel fits, as white taps into her involvement in the community, and white is also somewhat about law and conformity. Mirror Entity probably fits a little bit better still. From Starlight's perspective, she wasn't trying to reduce everyone to vanilla weakling creatures like Ixidron or Humility but was trying to create value through conformity. While the end result may fit those two cards, Mirror Entity (with more resources in the form of more mana) would fit what her goals probably were. Taking even weak creatures and making them bigger too. Following that line of thought further, Mirrorweave is probably the best fit to what Starlight was attempting to accomplish.

We saw by the end of the S5 premier that it was Starlight's own magic that removed cutie marks. We also saw in A Royal Problem that Starlight continues to be able to use magic that affects cutie marks by swapping the marks of Celestia and Luna. This feels like the sort of ability that ought to be on her planeswalker card as a loyalty ability. A single-target Mirrorweave effect is probably the easiest way to represent that, then just need to decide what sort of loyalty cost would be balanced for it.

Most planeswalkers have some kind of card advantage or pseudo-card advantage ability of some sort. Traditional blue "learning" style cards don't really feel like they fit her, so I'd lean towards red's "draw and discard" spells for inspiration there. She also doesn't seem like the sort to use destruction or direct damage effects. If we went with an aura theme, she might recover auras from the graveyard. I'm going to suggest that last one to fit with her ultimate. "Return target aura card from your graveyard to your hand."

If I wanted to imagine a Magic card which most represented the feel of how Glimmy acts, I'd say Warp World fits. Glimmy is to Equestria what Warp World is to a typical game of magic when it hits. I'd suggest that for her "ultimate." Something like, "each player counts how many permanents they control. Shuffle all other (not Glimmy) permanents into their owners' libraries, then each player reveals cards from the top of their library until they reveal as many as the number of permanents they controlled. Put permanents into play, the rest into the graveyard." The fact that Glimmy's controller gets a +1 free advantage in that due to counting herself, but not shuffling herself seems fine, and when paired with her second ability to recover aura's from the graveyard (in case her Warp World caused some to misfire), this probably makes for a fairly fun planeswalker.

Now on to the seven spells for the spellbook.

Lithl's suggestion of Spelltwine I feel is more on point if we want to examine her style. She has made use of things like mind control, time magic, and so on by mixing and tweaking spells created by others. She hacked the cutie map. All of that points to cards like Spelltwine, Magical Hack, and Mind's Desire as being appropriate to her style. I'd lean towards Magical Hack. Being a cheap spell to cast makes it the most "readily available" of the options, and it is the one most directly involved in changing the effects of another card, rather than a simple recursion/pairing of spells like the other two.

Auras also feel like they fit, in that she not only mind controlled her friends, but also enchanted Big Mac to cause him to be talkative, and so on. Much like her ability to manipulate cutie marks, she seems to prefer effects which enhance or diminish other targets. Crystallization feels like it fits not merely because it matches a thing she did during her fight with Twilight, but because it is an aura, diminishes the target, and the colors overall fit (blue, white, and green). Gift of Granite and Granite Grip both somewhat represent Starlight's encasing herself in crystal for both defensive and offensive purposes. Flight feels like a good choice as well. She self-levitates, and her hobby is kites. It does seem like she has a significant interest in flight.

I feel like Sasquame's (mostly humorous) suggestion of control effects to be a matter of confirmation bias. Even in the episode Every Little Thing She Does, Starlight may have mixed some control spells to do that, but Glimmy has hardly spent her entire onscreen presence mind controlling everyone. It was just one episode, dude. Don't let your visceral unease of the act overwhelm your perception of reality. Even in her former life as leader of Our Town, she didn't mind control the ponies, she convinced them to join her cause. Mind Control/Control magic hardly seem to be her signature and style, they seem more to be spells that she could, as a general unicorn, be able to cast and did so in an episode.

Welcome to the Fold fits her style way more. Be it through use of a literal mind control spell, or by simply appealing to logic or emotion and convincing people to her cause, Glimmy would be far more likely to go with converts and allies than she would thralls. And it alone on her spellbook is enough. Glimmy is far more diverse a character than would be represented by a bunch of control effects.

I would argue against including a time-related spell in her spellbook. Much like the control discussion, "it was one episode." She used someone else's spell, modified it, and used it to achieve her goal. She does not make use of time magic before or after that one event.

Imprint is a mechanic which I feel like overall fits Starlight's theme. Loading a spell into an artifact to modify it the way she hacked the Cutie Map (as well as what she pretended to do with a piece of driftwood) feels in-character for her. Isochron Scepter (to put Magical Hack onto) feels fitting. Spellweaver Helix feels very thematic of her as well. Spellbinder being an equipment feels the most fitting, as it also goes along with her "enhance creatures with auras" feel.

Starlight Glimmer (casting cost not determined yet, probably (W/U)(R)(R)(?))
Planeswalker
{?} Target creature becomes a copy of another non-legendary target creature until end of turn.
{?} Return target aura card from your graveyard to your hand.
{?} Warp World, yo. Slightly cheaty version.

My list:
Magical Hack
Mirror Entity
Flight
Crystallization
Welcome to the Fold
Gift of Granite
Spellbinder