The Shape Stealer is something of a conundrum for me. I can describe it in ways that make it sound like my dream come true: A story featuring a Master Vampire at odds with one of his creations, multiple time travelers including a sort of Earth-based Time Lords (though more like the Round Table) as the heroes try to fix damage that has been done to the timestream and thwart the time-traveling Master Vampire and his human colleague in their plans. I can also describe it in ways that would turn me entirely off of it and make me never consider picking it up: A story that treats explanations and time travel mechanics as such unnecessary window dressing that streams and dimensions are considered mutually exclusive concepts and the very idea of attempting to understand is considered a harmful exercise, causality plays a backseat to participating in a love triangle with oneself, and new powers and plot points are seemingly added at random and with only a slight possibility of being caused by the previous entry in this series.
In other words, The Shape Stealer features an aesthetic that appeals to one idea of storytelling while operating almost entirely in another. One difference that some cite as what separates Science Fiction and Fantasy is the fact that Science Fiction focuses largely on ideas, technology, causes and effects and tools, while Fantasy focuses largely on emotions, results, reactions and spectacle. On that spectrum, The Shape Stealer falls entirely in the realm of Fantasy. The entire story is built around the way things relate to Garet. Okay, there are some other pieces focusing on Marduk, but at this point I don't really consider dedicating a fraction to the book to the villains for the sake of world-building and raising tension as anything but good writing sense.
It's worth noting that none of the Marduk/Dee sequences really go toward explaining anything. They do give the first look at the new group of villains, who are essentially temporal anarchic terrorists that – like any other eco-terrorist – see themselves as holding the moral high ground, and are therefore just as motivated to destroy an ancient demon/vampire as anybody else. This adds dimension to Garet's story, but tellingly doesn't really explain the temporal landscape we're dealing with. It gives some understanding to why it's shifting, but trying to use that as an explanation of the universe would be like taking the statement that tectonic plates are shifting to teach a geography class. No, these sequences go primarily toward making the world stranger and more inexplicable, all the more to advance the idea that trying to understand the nitty-gritty details is futile and you're just along for the ride.
The other purpose the Marduk sequences serves is to build up Will, the love interests of this story. The 400 year old vampire Will Hughes has undergone a transformation to an 800 year old vampire who has lived the same history twice, which means he is better than everybody at everything, and more moral too. He is kept offscreen through much of the story, focusing instead on building up the mystery of what he does day to day – the mystery that is pretty much explored by Marduk.
Speaking of Will, he embodies a lot of the things that this story does that are generally considered the realm of bad fan fiction. By preventing his younger self from becoming a vampire, he essentially breaks causality for the sole purpose of having two versions of himself to form a love triangle with Garret. Some attention is paid to this, but it is mostly for the purpose of demonstrating that Garet must pick one version of Will to be with...eventually. Other than that, he is the perfect Mary Sue character. Not the self-insert one that the audience can relate with – that belongs to the Chosen One who randomly picked up the ability to read minds at some point – but the Tuxedo Mask figure who can always rescue the heroine at the appropriate moment to make him as attractive as possible. Despite losing an unquantifiable amount of his vampire powers for reasons that cannot be adequately explained if you apply logic, he retains his signature ability to never be harmed by anything ever due to the ability to control every atom in his body.
I described aspects of this story as “the realm of bad fan fiction”, and I mean it. More specifically, it is proof of why bad fan fiction will always be popular, no matter how the more detail-oriented writers of the world try to stamp it out. I'm not trying to say that this story is bad, although for me (and presumably a lot of people like me) it is incredibly frustrating. Still, there's something appealing about not focusing on the hows and the whats and focusing entirely on the way things affect the main character. After all, having a character with too many powers who faces no harm no matter what happens is no stranger to the genres of Sci-Fi and Fantasy; making it explicit is just cutting out the middle man. The same with the audience-identifying female character with impossible insight into the other characters. The specifics of how time travel works are seen by some as only a distraction from the “real” story – the way it impacts the characters. More than anything, The Shape Stealer emphasizes this approach. That's not to say that it wouldn't have benefited from a good editor focusing specifically on those aspects, but for what the goal of the novel appeared to be, that was the lesser priority.