I checked these books out from my middle school library. I remember the main characters were a girl and guy. They were both a sort of unexpected hero. I know it was set in a fantasy world perhaps on an island with a castle/kingdom nearby. I believe the bad guy was a magic user.
There were three books in the series at the time, possibly more since. I only read the first and third, sice somebody misplaced the second one. They were fairly new books at the time.
When I continued from the first to the third book I remember being a bit shocked that the main characters had gotten married.
The only other things I remember is that they used birds to relay messages, and maybe one of the titles had the word song or swan in it.
I think the cover on one was bright with the protagonists and a bit of the landscape and some bird.