I haven't finished this one yet, do we'll see.

I'm guessing you mean the first trilogy (apologies if I'm mistaken), as the First Law universe continues for 6 more books after that (plus a collection of short stories). After the Last Argument of Kings, there are 3 "stand alone" novels, which are the strongest, in my opinion, and then a second trilogy, which is excellent.

I believe there are more books planned for that universe, but The Devils is separate. The others I haven't read as they're Young Adult, so they probably have cheerier endings.

Were you to read on, you'd probably find some joy for characters you like, but I think unhappy endings outweigh the happy ones. Of course, I could be mistaken and you've read all 9, because the ending of the last can definitely be described as devastating too!