There are many resources for you to work from.
The Great Encyclopedia of Faeries, by Pierre Dubois, is a great book to reference. It's available at Amazon.
If you're looking for something more immediate, try the Working Encyclopedia of Faeries, a web resource at
http://enchantedrealm.dhs.org/faerie/fencyclopedia/fencyclopedia.htm
A more gaming-related product is Mongoose Publishing's
Fey Magic - Dreaming the Reverie, though this is more about their magic than about them directly, since its part of their Encyclopedia Divine series.
Another gaming product was
The Shadow Rift for 2E Ravenloft. It dealt with the Seelie and Unseelie domain of that dread realm. If you're interested in it, you may also want to look into its prequel adventure
Servants of Darkness.
Blood Spawn was a monster compendium for the old 2E Birthright campaign. While not totally sidhe-specific, many fey monsters are in it. It also somewhat details the Shadow World, the strange other-world where the fey dwell in that campaign, and the mysterious Seeming property that is both illusion and reality in that realm, and of course, it has several pages on the Seelie and Unseelie Courts of that campaign world. Since it was finished when that campaign was mostly done, it was released as a free web download. Download it here:
http://wizards.com/dnd/files/Blood Spawn2.pdf
The official Dungeons & Dragons website has a new monthly feature all about the fey. This month it deals with some basic information about the Seelie and Unseelie Courts, and special information on the thrones of the rulers there. Here's a link:
http://wizards.com/dnd/article.asp?x=dnd/fey/20021213a
Hope thats enough to get you started.