This is most definitely a novel written to conclude a trilogy, and it works well as that. It is not, however, the sort of book which you could pick up and read on its own. Well, I guess you could, but it would probably leave you pretty confused.
It's a suitable ending to the series; all the knots are tied off, Sabriel and Touchstone feature (something I missed in 'Lirael') and Lirael finds out exactly why she was never chosen to be a true Clayr. It's just not as good as 'Sabriel' - possibly because, as sadly often happens with book sequences, by the time the conclusion is out, your enthusiasm has waned rather.
If you can, read 'Sabriel' and 'Lireal' first - re-read them if you've read them once. Trust me, you'll get far more out of the novel.