Perhaps all the best stories have already been told and it is becoming impossible to formulate anything that is totally new. Certainly in this genre the derivations are always evident and it is only their recombination that is fresh. In this case we have the Greek myths, Faust and the Arthurian legends plaited together. A touch of Poe. A measure of Secret Garden. A smidge of Du Maurier. Some of the references are obvious, others obscured until the time is ripe for their revelation. Most of them are laid bare eventually (presumably on the assumption that readers wouldn't get them otherwise, which is a shame).
If that is a criticism, it's not one that matters, because once you get passed past any irritation at the early flash-back reverse-chronology structure and into the story proper, it is well crafted ripping yarn of a boy's descent into a world of magic and evil as he seeks to save an innocent. Amongst the battle between good and evil, there's a voyage of self-discovery, and a subliminal discourse on the nature of knowledge and power and corruption.
The human characters are realistic and sympathetic, even those who are by any definition downright odd. The uncanny creatures are sufficiently spooky, horrible, vile and violent. The landscape is alive.
Until…
Obviously, if you are going to write a trilogy and have determined that from the outset, then you have the structure of the three books in mind from the beginning. You know what links them. If you're of sound marketing mind, then you want to set up Book 2 at the end of Book 1. If the break is simply because the story is too long to get published as a single volume then you can end on a cliff-hanger. If the link doesn't take all of the characters forward, but only one or two of them, or (initially not even that), then there is a ""''shift"" '' that needs to happen. Sadly, this isn't always done with the degree of subtlety required.
''Advent'' would be a wholly more satisfying book if the final few pages had been left to the beginning of the sequel. They don't belong here. As I'm fortunate enough to have read an advance copy – maybe the author & and editors might think about taking them out – in which case these last few paragraphs may not make any sense by the time you come to read it. Which would be all to the good. It's worth the read.
For some of the myths that lurk behind Advent you might enjoy [[Troy: Fall Of Kings by David and Stella Gemmell]] and for the ultimate ring / quest tale clearly it has to be Tolkien.