Immediately following the events of Appletheosis, Applejack must relearn her place in the world as the new Princess Mara of Equestria. Meanwhile, her family and friends have their own troubles with the changes. Through it all, little Merry May, first in the service of her goddess, tries to hold things together and keep everypony sane and happy.
I won't tell you what to do, (this time) but reading Appletheosis first is highly recommended. The Shadow of the Goddess also provides useful background and will become very relevant in later chapters.