It works because Gemmell makes Hero in the Shadows move in interesting and unexpected ways. He does this in part by respecting his secondary characters. In most novels it's fairly easy to identify secondary characters when you first encounter them. Not so with Gemmell. Gemmell also has the courage not to spare his characters.
Many writers are reluctant to kill off a sympathetic or amusing character.
Gemmell dispatches them without a quiver. Nor does he feel the need to make such deaths heroic. Some of the most interesting characters die the way people die in real life. Hero in the Shadows may not be great literature, but it is an immensely enjoyable yarn. Gemmell gives the reader full value.