This is pretty vague, sorry, but maybe someone here will recognise this description.
It’s a fantasy book, probably less than 10 years old and more than 3, the first in a series. It is set in a non-Earth world. The main characters are the leaders of their people, lords and ladies type thing. They may or may not all have some kind of supernatural abilities. One character I remember in particular could fly. He wasn’t a noble himself, I don’t think, but was used by them as a messenger. He really loves flying, it’s his ticket to freedom. Most of the people were busy fighting a war, but the flying guy had to chase some lord’s runaway daughter, who had gone off to a big city and gotten in with a bad crowd. Flying guy rescues her by knifing a guy that’s attacking her in a graphic, gory scene.
I don’t remember much about the war, but it was against some known foe from the past, or another dimension, or a different race/species. Something about a valley and they wanted to flood it, or stop it being flooded, and the nerdy scientist types had to solve the engineering dilemma. There was also a lot of descriptions of the fighting, it wasn’t just about the engineers. There was a fairly big cast of characters.
It was around 300 pages, and I think the author was male. It was a tense, exciting book, and I wish I could remember the name, or the author, so I could find the rest of the series.
No idea, I’m afraid, but you might try asking over on Goodreads: there’s a group called What’s the Name of That Book??? which seems to be fairly good at tracking down half-remembered books, particularly fantasy and YA.
Is it one of the Codex Alera books by Jim Butcher? I’ve not read them (I gave up about 50 pages into the first book; it just wasn’t my thing) but what you wrote seems to match up at least partially with what I remember reading.
I was thinking that it sounded like one of the Codex Alera books, too. Unfortunately, it’s been a while since I’ve read them, and my copy of the first one, Furies of Calderon, is packed away so I can’t check it.