Reading the last two or three Dave Duncan books I've been struck by a common feeling; the author writes good, solid, rewarding books. His style is smooth, and never intrusive; there are no infelicitous phrases to stumble over, no names that cannot be remembered or mistaken for one another, no barriers to a happy reading experience. His descriptions of the society and the physical environment are extensive enough that the characters are not moving in a sterile world, and are spare enough that the reader is not bored by padding and bloat.
This is the first in an apparent series; the basic plot is not especially original. An oppressive dominant religion suppresses an older, more humane religion that welcomes and shelters gifted individuals. An attractive set of four siblings, adherents of the old religion, follow individual paths set by their distinctive personalities and gifts. Their actions plausibly follow from their personalities, which are deftly described, differentiated, and convincing, with a skill that is too often lacking in genre fiction.
The plot moves along steadily and clearly. The book is a good length, with enough in it to solidly fill a proper book without adulterating additives. The ending is fine, a bit open-ended, which results from its need to provide hooks for the next volume in the series.
I'm sorry to see that there are so few reviews for DD's recent books; he's been doing this for a long time, and his craft is thoroughly polished. He doesn't need to experiment, or flail around "finding his voice". He just reliably turns out one satisfying read after another, and "Against the Light" is another success.