There is one very distinct thing I remember from my first reading of The Creature in the Case: that I really didn't like it. At the time, I didn't really understand, or like, Nick. I didn't like that the story wasn't in the Old Kingdom, and I didn't like that it was so short and that Lirael only appeared in the last five or so pages. As always though, my lure for Nix's writing style won me over, but that was about it. I didn't have much care for it at the time.
I've re-read the series countless times since, and last year I picked them all up again for the firs time in two-three years, and then when I finished, read this one. My opinion was drastically changed, I must say. For the first thing, I really like Nick a lot more than I did before. If you like him, it's a great opportunity to see inside his head after the events in the Old Kingdom and at the Wall, how he's adjusting to realising that in his world, science isn't the simplest answer to life, and also to see how he deals in a crisis. He becomes a sort of hero after his misadventures last time, which kind of lead him to redeem himself. I must also admit that it's a draw that he was someone perfectly normal, like the reader, who was drawn up into all of this, so you can place yourself on the same level as Nick himself. It's interesting to see how the Dead effect other people aside from those of the Abhorsen lineage who have the power to get rid of them by bells at the least. Nick doesn't.
I must also admit, that I'm now more of a fan of the relationship between Lirael and Nick. I've always loved Lirael, but it's nice to see the slight embarassment she feels around him and some of the things they say to each other.
It's also, quite short. It feels more like three scenes in a movie - the discovery of the creature, the creature attacks, Nick saves everyone (just about). Despite how short it is, it was enjoyable and never taxing. Even if you're not really keen on the premise, I would read it regardless, given it's only just over 100 pages. It's a nice tag on, a sort of epilogue to some of the events in Abhorsen that were left unanswered.