In the SR4 FAQ, the answer to several questions about magic is "This question will be answered fully in an upcoming sourcebook."
The questions unanswered include the following little wonders:
If you enchant a car as a focus, can you run someone’s astral form over with it?
Can you attack a target on the astral using a dual-natured metahuman or critter as an improvised melee weapon?
It doesn't seem to be Street Magic, or any of the "main" supplements, as they are mentioned in the FAQ, and it was updated long after Street Magic or the other books were published.
Does someone know what book it refers to?