The Redwall series by Brian Jacques: Talking animals, delicious descriptions of food, and a huge universe of over 30 books to explore that don't have to be read in any particular order
The Inkheart series by Cornelia Funke: Although they're only a trilogy, each of the books in this series is the most exciting tome you'll ever read. Fun facts: these books are some of only a few middle grade books in translation and if you love it her Thief Lord and Dragon Rider books are just as great!
The Unwanteds series by Lisa McMann If you've got a kid who's ready for the dystopian stuff, this is the place to start! Kids who have "unwanted" and dangerous artistic talents are banished--but these creatives just might be creating an uprising all their own.
Pierce has written dozens of books full of knights, spies, courtiers, and magic, but this was the quartet that got her started and remains one of my favorites to this day. By forging a letter from her father, 10-year-old Alanna takes her twin brother's place at the palace to become a knight; hiding her identity turns out to be only half as hard as her training, however!