First, this series is what I would consider a Young Adult fantasy. There are two nations, separated by a huge wall that is nigh impassable except under very specific circumstances. One one side, The Old Kingdom, the rules are governed largely around what is known as The Charter, the magic system. Here Charter mages (the good guys) are pretty much always hunting the users of Free Magic (basically the same magic, but unrestrained. The evil guys.) Free Magic users usually use their corruption for necromancy, as in bringing back zombies for their evil will. There is a monarchy, and the magic doesn't allow for technology here. South of that, across the wall, is Ancelstierre. This nation is sort of like nineteenth century America, I would say. They have technology such as cars, guns, planes, etc. The people here are pretty much us. They don't have magic and don't believe in all the nonsense they hear happening on the other side of the Wall.
So the plot is very linear, the characters are sort of cookie-cutter, and there is quite a lot of 'telling' as opposed to 'showing'. (Ex: Lirael knew the man wanted to run.) I do find fault in all of this, but they are the sort of things a younger audience would forgive more easily than an older one would, so I let it slide. Basically everything happens as expected, but that doesn't automatically make it bad.
The thing I liked most about the series doesn't actually have anything to do with the writing or the books. Mostly, I enjoyed the world that this was set in. The premise of these two vastly different nations interested me, and the river of Death as it is depicted in the books is awesome. I liked getting the sense that everything in the Old Kingdom had an ancient and grand purpose. The Wall wasn't made just because those two nations were at war. The Wall was made for a reason. The line of Abhorsens are for a reason. In fact the entire Charter was made for a reason. You sort of get that sense in the first book, and while I actually didn't like the first book (I kind of hated the protagonist) the series redeemed itself by the end of the trilogy.
I'd recommend the series to a younger audience. Anyone who likes fantasy and death would really enjoy it. It has zombies in it, yes, but stating that fact gives off a very misleading impression. This isn't a zombie apocalypse. I would pitch it more accurately as a fantasy series where the magic manifests itself very closely with death.
It's not an amazing series, but a young reader would probably get a lot out of it.