This is YA fantasy that earned its bestseller status—it's genuinely engaging with a protagonist who has agency and brains. The assassin angle gives it edge without being gratuitously dark in book one, though there's real violence and death throughout.
The 14+ age rating is spot-on. This isn't a book you hand to a 10-year-old who loved Percy Jackson. It's darker, more violent, and morally complex. But for teens ready for that next level of fantasy, it delivers: mystery, political intrigue, romance, and a heroine who can handle herself.
The series gets more intense as it goes (and longer—later books are 600+ pages), so consider book one a test run. If your teen loves it and handles the content well, they've got seven more books waiting. If they're disturbed by the violence or the moral ambiguity of rooting for an assassin, maybe shelf it for a year or two.