Enjoyed it quite a bit, however, as a stand alone book I think it is too weak right now, and needs some rewrites.
1: It needs to be refined with "show, don't tell," as right now there is a lot of exposition. Good and interesting exposition, but I think two or three rewrites focusing on how to show some things instead of telling them would be good.
2: Character development is there, but a bit rough. Major elements include Samael forgiving Knuckles, controlling his anger,, and his introspection about his faith and the debt he owes his parents, guardian, and heaven. That needs polishing. Including being able to depict his changes more outwardly, with less inner monologue. Understand, inner monologue can work, but not as well for Samael, as he comes across as a very direct man who lives his life in the open. Additionally, while the character development for the other characters exist, it needs expansion and polishing.
3: Jessy in particular (and to a lesser extend Camilla) need more development. Jessy in particular feels disconnected from the story. Frankly there is currently no reason to have her in the story at all as anything more than to provide the summoning of a flier to pull the floating group. Which could easily be fulfilled by an unnamed summoner. If you intend to give Jessy more to do later than I would suggest instead having her only appear as the cameo on the trip to the Bonepit and then again to summon the flier. Then have a more connected and detailed story of Jessy's training to start off the next novel. Perhaps making her the POV for that novel. Camilla on the other hand does begin to get filled in, but perhaps because we see things so much from Samael's POV (and he's an oblivious man) it takes a while before we start understanding her. Something that might help with Camilla is answering the question of why she doesn't already have a good adventuring group despite being so powerful. Perhaps building on the fact that she's a bit of an airhead, sheltered, and possibly people not liking that she's the clan leader's sister (maybe a feeling that she didn't work her way up from the ranks). Something that adds more to the mishmash of misfits that Samael's party seems to be.
4: There should also be some rewrites based on improving pacing. It felt sometimes like you skipped over "boring" parts, which isn't bad, but a better approach would be to take some of that time to develop the characters more. It just felt sometimes like the story was trying to "get to the good stuff" instead of naturally flowing there. Pacing rewriting should probably be done before other rewriting efforts.
5: There also might be some worth in developing the world atmosphere more. In the beginning there was a real oppressive feeling about the injustice and corruption of the world. But then we get to the dungeons and a lot of that isn't felt anymore. Something that subtly impresses that the corruption of the Duke is omnipresent (even if more diffuse in the adventurer world) would be fitting. Especially since the Duke ends up the main villain. Nothing explicit or in your face, I think the threat should feel lessened, and more distant, but something more than there currently - something that says it may be distant but not gone. Also, one thing I noticed was after the guardian died, there were a group of people who showed kindness to Samael, referring to him as "our church boy." Maybe a small earlier scene signalling that there was some of that would be good. Something like a mother figure. A
Nancy figure, yes, I think Nancy would be the best fit for this story. Nothing major, just a small scene prior to the later scene after the priest dies.
Probably some more things could be improved, but I think that's a good starting list.