I liked this book much better than the first one, The Book of Three. I think that is it is because the characters are more grown up and less annoying (specifically Eilonwy, she is not as combatative). Taran, the main characte , has some great character development in this one from the first book. 
One thing I liked about this book is that because Taran is more grown up (or is growing up) the book is darker than the first. The main premise is that the people of the realm is going to steal a cauldron that the basic king of the underworld uses to raise the dead, using dug up corpses! I was not expecting such a dark concept to be in a book for middle schoolers (it isn't too perpetrated on but if you catch on to the implications, it's dark).

 I put misogyny and sexism in the content warnings, but there is a few here and there sexist comments towards the female main side characters, Eilonwy. They're always along the lines of "you should go back to working in the scullery" or "I shouldn't take advice from a girl." They are kind of just casually said to her face 🙄 but can you expect anything more from a book written in 1965?