Mother of Learning is a story I'd seen recommended on /r/fantasy a few times. In the first chapter, we're introduced to the protagonist, Zorian Kazinski. He's a budding mage who is returning to the mage academy for his third year of schooling. He is a diligent student, but is held back by poor "mana reserves" and a somewhat prickly personality.
Around a month into the school year, the city hosting the school is attacked by a force of monsters and Zorian witnesses a fellow student (Zach) fighting an incredibly powerful lich leading the monsters. Zach makes some cryptic comments and the lich casts a mysterious spell, and then Zorian wakes up in his bed at the beginning of the month.
Zorian quickly figures out he is caught in a Groundhog Day-style time loop, living through the same month over and over, and only he, Zach and a mysterious enemy appear to remember what is happening from month-to-month. He sets out to learn as much magic as he can, both to figure out what is happening with the time loop and to stop the invasion of the city.
This story is an interesting story of Zorian learning to be both a better mage and a better person in general. It is somewhat long-winded (my e-reader app estimates it at 2700 pages), but while I was reading it it always felt like he was progressing towards a goal. It also felt like the setting was designed for Dungeons & Dragons, and in the author's notes at the end he confirmed that the story started out as a way to flesh out a D&D setting.
I enjoyed it, but I'd be hesitant to recommend it given how long it is.