There are three main "shadow" prestige classes for spellcasters: 1) Shadow Adept (Player's Guide to Faerun); 2) Shadowcraft Mage (Races of Stone); 3) Shadowcrafter (Forgotten Realms: Underdark). There is also a Shadow domain from the Eberron sourcebook, if you want to go the divine route (probably good in combination with the Trickery domain or Gnome or Illusion domains, if you can combine setting books).
Dragon Magazine Issue #325 has a few articles on the subject as well, with some new shadow spells and new shadow feats (inlcuding a very useful feat called I think Enhance Shadow Reality). Dragon Magazine Issue #322 has an article on the Shadow Apostle prestige class (which a pretty decent one as well). Dragon Magazine Issue #291 has a lot of articles on Gnomes, which work best with the Shadow Spellcaster classes.
The most common build I have seen that focuses on shadow spells is Gnome Illusionist 5/Shadow Adept 1/Mindbender 1/Shadowcraft Mage 5/Shadowcrafter 8 (more commonly known as Snow's Killer Gnome, as it was first written up by someone named Snow Savant on the WOTC boards, see link below). Generally, people take Shadow Magic (gets you into the single level of Shadow Adept), Spell Focus: Illusion, Greater Spell Focus:Illusion, Heighten Spell, Spell Mastery (or the Illusion Mastery variant from Unearthed Arcana), and Signature Spell: Silent Image or Minor Image or Major Image. Some people are big fans of Earth Sense and Earth Spell (both from Races of Stone) in later levels for this build as well.
The purpose of Snow's Killer Gnome is to have tremendous versitility in choices of spells to cast, and a very high DC for your spells (something that cannot be easily done in 3.5).
The third level ability of the Shadowcraft Mage prestige class is key to this build. It allows you to convert any of a number of illusion (figment) spells into shadow spells that function exactly like the shadow conjuation or shadow evocation spells (but with a more beneficial "reality" percentage). In other words, you convert these figment spells into ANY evocation spell on the wizard list (even those that are not in your spell book, on your spell list, or even those on your list of prohibited spell), or any Conjuation: Creation or Conjuation: Summoning spell (again even those not on your list, in your book, or on your prohibited list). And, the converted spell uses the DC for your illusion spells, not for the evocation or conjuation spells. It also uses the componants (casting time, verbal componant, material componant) of the illusion spell (which is standard action, silent, and none with this build) instead of the componants for your evocation or conjuation spells (which is sometimes a long time, with verbal componants, and expensive).
The rest of the stuff all adds to the mechanic I mentioned above. The Heighten Spell & Signature Spell feats are both used on the single Figment spell of your choice (probably Silent Spell). You spontaneously cast the figment with Signature Spell, Heighten it to the spell slot level of your choice (this will become a full round action, as the rules state applying a metamagic feat to a spontaneously cast spell is a full round action, and that rule applies here as much as it does to sorcerors), and then convert it to the evocation or conjuation :creation/summoning spell of your choice. Spell Focus, Greater Spell Focus, Shadow Magic, and the Gnome +1 DC to illusion spells all apply to the converted spell (as it is still an illusion). The +5 to spell resistance penetration from the Shadow Adept all apply (as it is still an illusion). The resistance to dispelling and identification from the Shadow Adept all apply (as it is still an illusion). The Silent Illusions ability from the Shadowcraft Mage applies making these shadow spells all cast silently for no extra cost (as it is still an illusion). And finally (though some have argued this), the enhanced reality abilities of the Shadowcrafter ad to the reality of these converted spells. Earth Spell makes your Heighten Spell metamagic cost 1 less and increases the caster level of the spell (which is a big help, if you go that route). You also have a lot of fun ancillary abilities, like 4+Int skills for Shadowcraft Mage, constant 20% miss chance against attacks due to a shadow cloak surrounding you (which you can also hide in, something a Gnome can do better than others and which is a class skill for many of your levels), and Telepathy (if you choose to go the Mindbender route), which is quite useful if you summon cretures with your shadow magic (able to command them if they have intelligence), want to trick opponants into thinking your illusions are real, want to serve as a go-between for your rogue scouting ahead to the rest of the party, etc....
It's a great build. Not overpowered, but fun to play and flexible, and all with shadow spells.
The thread that started it all:
http://boards1.wizards.com/showthread.php?t=290914&page=1&pp=30
