The promise started in Sabriel is kept in the second book in the trilogy, Lirael. The book released April 11, 2001.
I had the good luck to pick up Lirael merely a day after reading Sabriel. Readers who read the first book back in 1995 may well want to re-read Sabriel to fully immerse themselves in the world Garth Nix has created.
That's not to say this second book does not stand on its own. Like Sabriel, it is a remarkable fantasy that will engross you completely. However, readers will find that questions brought up in the first book are answered in the second one and a re-reading of the first will add to your enjoyment.
Lirael is a daughter of the Clayr, a clan of people who can see into the future. They are one of the founding bloodlines of the Old Kingdom.
But Lirael is different from the other Clayr. She is dark where they are light. And she cannot See into the future as they do.
She despairs of ever truly belonging in the world of the Clayr and seeks consolation in the Charter (the controlled magic system of the Old Kingdom) and in creating the Disreputable Dog.
Across the kingdom, events are taking place that will ultimately change Lirael's life forever. Prince Sameth, the son of Sabriel and Touchstone, helps awaken her to her destiny (along with Mogget, who readers will remember from the first book) and to her true place in the Old Kingdom.
This book is a great follow-up novel. It strengthens the story and our understanding of the Old Kingdom. Myself, I can hardly wait until the third book comes out: The Abhorsen.