I also recommend most of the sequels: The Circle Opens, another quartet where each of the four takes on their first students; The Will of the Empress, what happens after they return from their travels in the second quartet; and Battle Magic, which follows Briar's group on the return trip before The Will of the Empress, when they got caught up in a war.

I wasn't really a fan of Melting Stones, which mainly focuses on Briar's student and takes place at the same time as The Will of the Empress. If I remember right that book was also an experiment, written as an audiobook first then later released as a regular book, so that may explain why it doesn't feel as good to me.

The author has had another in mind for years too, a continuation of the problems Tris had been having from The Circle Opens onwards: she can't figure out how to make a living with her magic, because she refuses to use it as a weapon. Unfortunately, the author doesn't actually have the rights to this universe, so she has to wait for Scholastic to want another book.