I went into this feeling really hopeful (D&D magic school? Yes please!), but what I was left with was a hodgepodge of half baked ideas that tried to do lots of things rather than a couple of things really well.
The Owlin is a fun inclusion, but it's strange that this is the only player option (beyond the spells – I'll get to those). It feels like this could have been a great chance to add some subclasses that would fit into a magic school. Certainly, a few more magical subclasses for martial characters would have massively opened up options for what to play as during the campaign. The spells are… Fine? Silvery Barbs isn't nearly as game-breaking as some make it out to be, unless you're building around it with something like a Divine Soul Sorcerer with an Order Domain Cleric dip. Mostly I'm just disappointed that the book about learning magic really didn't have that much new magical stuff to learn.
The adventure is probably the most disappointing aspect though. As a DM, I want an adventure that makes my life easy! I want to be able to read the books, maybe add some unique hooks based on my players' backgrounds, and then jump in. I don't doubt that someone could make the adventure included fun… But there's just so little to work with in spots that it would take a lot of work.
If you want a good MTG 5e setting book, I recommend Mythic Odysseys of Theros. If you just want a good setting book, I also recommend Theros. If you want a 5e magic school campaign, I recommend homebrewing something yourself, because that's what you'd basically be doing anyway if you buy this book.