18-year-old teenager discovers that his father has a secret past and is sucked into Tir Na Nog - the Irish / Celtic land of Faerie, where he has adventures.
As plotlines go, that is one that can work very well for me. Neil Gaiman's Stardust is one of my favourite ever novels. Alas, Shadowmagic is no Stardust. Instead, our teenage narrator is permanently wise-cracking, but never really funny. The adventures mostly consist of him meeting people, who instantly try to kill him, and him observing that everyone seems to try to kill him first, ask questions later... Oh, and there's some humour at the way he reacts to first meeting his mom (who looks like she's in her 20s and who's very hot...)
The plot moves at breakneck pace. It has to. There isn't much else to this novel than plot. The twists are not very surprising, and the story never manages to generate any tension or investment in the characters. Instead, you just sort of read along because things move quickly, smoothly, and well-lubricated with semi-amusing asides. It's the literary equivalent of a McDonalds quarterpounder: just about the least offensive and most easily wolfed down thing imaginable, but ultimately unsatisfying.
Read this if you love the books of Esther Friesner, or K.E. Mills' Accidental Sorcerer - this book falls into the same rough territory.