This is a promising debut, a nice gloomy fantasy which builds on an old SF metaphor about immortality and sterility to give quite an original feel.
It begins conventionally, with the traditional importation from our own "real" world of a useless-seeming saviour (Eleanor) who's not much of a person but is clearly destined to end the unchanging stasis that freezes the fantasy domain. Jones then conscientiously tangles the often too-simple issues of good and evil. Most people on the "wrong" side are likeable, perhaps pardonably selfish: contrast Western civilization and the suffering Third World. The moon-goddess on the "right" side destroys an innocent bystander in a shockingly gratuitous way, merely to underline her authority. Few characters are certain that they're doing the right thing, or that if they are it'll lead to the right end. Gruelling examples are given.
Good stuff, though still very much a first novel. Jones appears to have learned valuable complexities from Stephen Donaldson but has also grown over-fond of one of his pet words, "exigent". The giant telepathic hawks are well evoked but with rather too close a parallel to Anne McCaffrey's dragon "Impression". Several chapters end with somebody dying melodramatically only to re-emerge later as not actually dead. While beer, sandwiches and rucksacks might be universal, I'm unsure that fantasy worlds are enhanced by jeans, cigarettes, whisky, and digitalis (why not "foxglove"?). Problems of transition between worlds and the fact that both use English are fudged with quasi-scientific waffle about dimensions and resonance: a wiser policy might have been, "Never apologize, never explain."
These are quibbles. There's talent here, and Fly By Night ends well despite impending trilogy sequels.