First off, for those who care about those kind of things, Age of Assassins is the first book in a trilogy, with the next (Blood of Assassins) due out next year. Secondly, although I enjoyed this book quite a bit, I didn't love it as much as some people I know did and I've been trying to figure out just why that is - more on that later...
Age of Assassins is told from the point of view of Girton, a teenage boy with a clubfoot who just happens to be apprenticed to a female assassin called Merela. We first meet the two of them as they are sneaking into a castle to find out what they've been hired to do, only to discover that Merela has some personal history with the person who's hired them and that what they've been hired to do is not to kill someone but instead to keep someone from being killed. For that, Girton needs to infiltrate the castle as a member of minor nobility while he and Merela (disguised as a jester) try to figure out who the other assassin is (and ideally, who hired them).
All this is set within a world where anyone possessing magic is ruthlessly hunted down and their blood used to try and stem the results of the magic wielded by a previous rogue sorcerer. I found the overall worldbuilding much more accomplished than the dynamics going on within the castle itself - the corrupt prince they've been hired to protect was just a bit too much of a moustache-twirling villain to be anything but one-dimensional and I couldn't see why exactly Girton and Merela should work that hard to keep him alive. Likewise, the moment a female stablehand is introduced as one of the few female characters Girton's age, it was pretty obvious she was there as (doomed) relationship fodder.
And that, I think, along with a general lack of caring about teenage angst, was why Age of Assassins didn't quite work for me. Well thought-out overall worldbuilding let down by much more stereotypical relationship shenanigans closer to hand. Token female love interest who sees beyond Girton's disability - check. Character who is not who he seems - check. Trusting relationship that will probably come back to bite Girton in the next book - check.