I actually finished reading this a few weeks ago but I had so much else to write about on the blog that I never got around to posting about it.
As I wrote previously, this two-volume story starts off simultaneously compelling and frustrating. So many of the plot turns in the first volume make you want to scream at the characters to do something different. But it occurs to me that this seems like a criticism I've heard pointed at a lot of TV shows, and it's not the same.
The best example is Heroes, particularly in seasons two and three. Several of the characters got so powerful that the only way the story could proceed without them simply mopping up any problems is if those characters also got really stupid. Not walking-in-front-of-a-bus stupid. More like never stopping to think about problems and available resources to solve them, plus not having much common sense, plus being easily fooled again and again by the simplest subterfuges.
The characters in Mordant's Need are also sometimes dumb in frustrating ways, but in more understandable ones. They have good reasons not to share information with each other: sometimes when they do, bad comes of it, and they don't have the same information we do. They are kept too busy to investigate things. And they are human... and this is the part that's hardest on the reader: we can't help feeling enough detachment to think, "you've got a half-day free, why not spend it experimenting with such-and-such". But if you were in the same situation you might spend even more time just dealing with the emotional impact of the situation and not being very productive. Sometimes the author fails to convey that adequately. It's a difficult line: spend too much time exploring the real human reaction to these extraordinary events and the characters come off as whiney; spend not enough and they come off as inexplicably ineffectual. In book one, they do both. And it's all very realistic but still annoying... at least until it starts to build up a payoff.
In the second volume, this factor starts to ebb as the characters get more sure of themselves and their situation, and the mysteries of "what's really going on" begin to fall more and more into place during the build-up to the resolving conflicts. In a way, all that ineffectual bumbling pays off by making decisive actions seem more impressive when they come. There are a few points along the way where things seem like a let-down: in particular, there's a lot of build-up about what the Congery has planned that turns out not as much as one expected. But the resolutions when they come are even more satisfying than I remembered from my previous reading. This book has most of the good parts of Donaldson's better-known Covenant series while remaining infinitely more accessible. (And incidentally, it's a lot sexier.)
The following is a minor spoiler about how Imagery works, so if you're reading the book, you might want to stop here.
I've been thinking about the problem of translations within the world of Mordant. Geraden early on comes up with the idea of translating a flat mirror into a curved mirror to get around the prohibition against flat translations. Later, we learn this isn't quite workable because flat mirrors translated into curved mirrors turn to dust. But Eremis has found an oxidate which allows it to work.
If you didn't have that oxidate (and it's possible the secret was lost), it seems you could do the same thing. Just translate a kiln into a curved mirror depicting a bland, unthreatening landscape, then translate in supplies and finally translate in yourself and make the flat mirror there. All this would take is a flat mirror you'd already made showing somewhere interesting (so you could know how to copy it), and someone else with a curved mirror showing somewhere harmless.
But if you made a flat mirror in that other world, would it still show the place within Mordant it showed when you made it in Mordant? If it's a flat mirror shouldn't it show someplace within the world where it stands? For that matter, would the flat-inside-curved trick even work at all as it did in the books: wouldn't the flat mirror, once translated, start showing a different place, somewhere inside the world where it was sent?
All this suggests a simpler solution anyway. Once you have a curved mirror showing a safe, non-threatening place, just build your fortress there. You can safely translate your workmen, your supplies, and your armies there. Once there, make curved mirrors until you have ones showing important spots back in Mordant, and attack through them. Not only do you entirely avoid the whole flat mirror issue, you also are far more unassailable than you were at Esmerel: after all no one can march an army towards your fortress. No one can get to you save through the mirror your confederate used to get you there in the first place, or another one like it. Even accounting for powers like Terisa's and Geraden's, you'd be almost unstoppable.
The only way to avoid this problem is by author fiat: for some reason you can't make mirrors in the other worlds that show Mordant, though mirrors brought from Mordant still work there. But this seems way too arbitrary. If mirrors work differently in that world, the flat-inside-curved trick probably shouldn't work anyway.