I rather enjoyed Lirael. It was a fine continuation of the story begun in Sabriel with the introduction of new characters and the re-introduction of old characters such as the Abhorsen and the Prince. You cannot exactly consider this book a normal continuation of the storyline. Where Sabriel is on its own one book and one plot this story is set afterwards along a different plot yet with some ideas drawn from the original book. That said it was still readable on its own.
Lirael is the story of a daughter of the Clayr, Lirael, and her journey to understand why it is that she does not belong among the Clayr. Unlike the others she is extremely late in developing the desired Sight. To ignore this she becomes a librarian in a magical library where old Free Magic objects exist alongside Charter Magic. It is also the story of the young prince Sam as he struggles to live up to his destiny to become the next Abhorsen.
It was a fun story let down by the fact that it ends in a cliffhanger and the fact that I have no time to go reading Abhorsen at this moment. That said it was a clever tactic to make people read on even if it was frustrating. Unlike Sabriel it is clear that Lirael and Abhorsen directly tie into one another with events taking place in Abhorsen just after the conclusion of Lirael. Hopefully the conclusion ties up information about the Charter and the Charter stones. Still if you read Sabriel this is a worthwhile sequel and I do recommend it. I'm just warning you in advance about the cliffhanger ending. Also if you have not read Sabriel I fully recommend that you do.