Maggie Stiefvater is the author of several YA fantasy novels but it is probably best to describe her as an artist because she does not only write novels but also illustrates children’s books, tarot cards and writes music for each of her audiobooks.
Her most popular series is the Raven Cycle (TRC) where she tells the story of a group of friends who are on a quest to find a medieval Welsh king who is supposed to be sleeping underground in Virginia. This might sound very weird at first but the series is full of references to Celtic mythology and also features some Arthurian elements. On top of that you will encounter interesting characters, friendship, a magic forest, dreams, psychics, prophecies, mirrors and evil Latin teachers.
The Raven Cycle consists of The Raven Boys, The Dream Thieves, Blue Lily, Lily Blue and The Raven King but there is also the Dreamer Trilogy set after TRC. The first instalment, Call Down The Hawk, was published and it is a series which is featuring some characters from the Raven Cycle but focuses on the Lynch Brothers and many new characters. It can be read without knowing the previous books but it contains a few spoilers so you might consider reading TRC first.
Other books by Maggie Stiefvater:
The Books of Faerie (2 books): Celtic faerie mythology, four leaf clovers, music
The Scorpio Races (standalone): Scottish kelpie mythology, a race with dangerous water horses
The Wolves of Mercy Falls (Shiver trilogy): werewolf romance (very cliché YA fantasy romance)
Sinner (spinn-off standalone): werewolf romance but focusing on a different character of the Shiver trilogy, werewolf and former rock star on a reality TV show
All the Crooked Saints (standalone): Mexican mythology, strange magic/miracles, saints, overcoming your own demons