Tome of Magic has the Shadowcaster (I think?) uses 'mysteries' - essentially shadow magic.
Planar Handbook/Manual of the Planes have Shadow Plane stuff, including the Shadow Creature Template. There's a template that's a basically a toned down Shadow Creature somewhere (Lords of Madness, maybe? Not sure).
The Forgottern Realms Campaign Book & some other supplements like Player's Guide to Faerun has Shadow Weave stuff, which may or may not be the sort of thing you are looking for.
Also see the same for details on Shades (race, city of, etc)
FR's Underdark has shadowcrafter PrC, Races of Stone has the shadowcraft mage PrC, Book of 9 Swords has shadowhand maneuvers & the shadow sun ninja PrC. Player's Guide to Faerun has the shadow adept PrC (user of the Shadow Weave, that smootrk mentioned for FR), Complete Divine's web enhancement has the darkcloak PrC.
I noticed the Unearthed Arcana, P63 Illusionist varriant, referenced 'shadowstuff'.
I'd love more information on what it is, and what the properties are, but it seems to be a vague and varied concept that depends on the writer. I'd also like to know if it was described in 2e or AD&D.