Symbolism, Imagery, Allegory
When something happens in the shadows, it seems a little more mysterious than if it happens in broad daylight or under a slew of hundred-watt light bulbs. So when you notice a lot of shadows in a book, you should think twice about what might be going on, and this book has a lot of shadows.
From descriptions of buildings from afar "looking like no more than the shadows of a larger city" (2.4), to the assertion that Jamie's demon's mark has "torn edges […] black as shadows, black as blood in the night" (2.132), we can see that shadows are associated with hidden or sinister things. So when we see shadows associated with particular people, we might start to wonder if there's something secretive or sinister going on.
Mum, for instance, is often accompanied by shadows. When we first encounter her, she's standing in a doorway, "her magicians' charms shining with power, her hair falling like shadows over her face" (1.52). And later, as Mum comes out of the house to get in the car, we read that, "Mae and Jamie's faces suddenly changed, as if a shadow had fallen over them. Nick turned to see that shadow was actually Mum's dark form" (3.99). A little further down the page, Mum's "black flag of hair streamed behind her as she went, as if it wanted to cling to the shadows" (3.100).
All these instances together suggest that either this lady is a little sinister or she's got something to hide, and eventually we realize both are true.
But Mum's not the only one who's often seen with shadows. When Alan is in the midst of his plotting and planning, he looks up at Nick "with dark troubled eyes, blue under shadows" (4.65). And when Nick sees him at the breakfast table one morning, Alan looks "pale and worn as old bone [… and] there were violet shadows under his eyes" (6.76). It makes sense that the guy who keeps so many secrets would also be cloaked in shadow from time to time, doesn't it?
Also, when Nick looks at his own face in a crystal ball at the Goblin's Market, he sees something resembling "a shadow falling over a lake, a silhouette that grew more distinct, moving from a shadow into the lines of a face" (6.34). Dun dun dun…
With all these shadows (and trust us, these are just the tip of the iceberg—there are plenty more), Rees Brennan is letting us know that there are a lot of things lurking beneath the surface in this book and that there are plenty of secrets to be uncovered. Because lots of different shadows appear in lots of different contexts to convey this message, we would say that shadows are a motif in The Demon's Lexicon, not a symbol.