Faerzress sure is a part of pre-4e Realms. Its effects are discussed in 
Underdark, pages 48-49. There's very little lore on it there, though. (Big surprise, since it's 3.5..) You'd probably have better luck looking into 2e books and novels. The exact effects of the radiation, and what it has to do with Drow magic, has been contradicted a few times though. I think. I blame most of the Drow confusions on Salvatore, who doesn't know crap about the world he's been writing about for over 20 years.
I don't know where to look. It's not mentioned in 2e 
Drow of the Underdark. I didn't find anything in 
Magic of Faerûn either, but I didn't look very carefully.
Drizzt Do'Urden's Guide to the Underdark states the following:
Quote:
The Weave has undoubtedly played a crucial role in the Deepearth, for vast cavern networks suddenly appear from time to time, leaving pockets of Underdark radiation, known to the drow as faerzress, in their wake. And races flourish.
The book also discusses the radiation's effects on magic (teleportation, divination, the usual) and states that faerzress-imbued rocks emit light much like the aforementioned lichen and fungi. (Which might not have anything to do with magic.) Then, the following is stated:
Quote:
Drow wizards and priests are known throughout the
Underdark and surface world for mass-producing unusual
weapons and clothing with quasi-magical properties. Such
properties are partially the result of exposure to faerzress, a
powerful and strange radiation found only in the Underdark.
Drow societies live and die by the proximity of
faerzress to their cities.
Some locations of strong or particularly weak radiation are mentioned throughout the book (remember Zip's lair from the 
Shadows and Starlights trilogy?), after which Sshamath's history is told:
Quote:
The defining event in Sshamath’s history was the collapse
of its most precious resource: the Underdark radiation
known as faerzress. Localized appearances and
disappearances of faerzress are not unheard of. These
unexplained fluctuations typically mean the collapse of
nearby drow city-states or the establishment of new ones,
as the dark elves migrate to new sources of the radiation.
By -2872 DR, five years after the problem was initially
detected, Sshamath’s defenders were bereft of drow magic.
The city was on the brink of anarchy.
So we know it comes in pockets, it affects magic for good and ill, it's important to drow and other Underdarkers, and it has some effects on natural environment. Where it came from, how it feels like, how it's harnessed, how it affects spellcasting, what kind of mutations it's likely to cause; all this we don't know.
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On Joon, Kjetta wrote:
 The guy that probably has sexual fantasies about masturbation. I mean, Iron, you're a bookworm nerd that even in your wildest escapism fantasies flee to the internet to play the role of another bookworm nerd? Come on!