Book wise, there's "The Seinfeld Universe" by Greg Gattuso, pretty good.
The Entertainment Weekly "Seinfeld Companion" is a decent one, although it seems a bit short.
I think th ebest is "Seinfeld the Totally Unauthorized Tribute [Not that There's Anything Wrong with that]" by David Wild.
My personal favorite for for little Seinfeld tidbits is the special issue of People magazine. I bought a lot containing all this and more on ebay for about $30, so if you want some good memoribilla type books, check out ebay.