I just wanted to comment that written Cantonese isn't that rare (sure, compared to written standard Chinese it is less common, but rare?).
Magazines like New Monday (新monday), Sudden (忽然1周), Yes, etc. are almost entirely in written Cantonese and all of these magazines are weekly--that's hundreds of pages of written Cantonese per week :)
Magazines like Touch (東Touch) or Next (壹週刊) also contain some written Cantonese, and even the little daily free newspaper they hand out in the HK MTR has a few articles in written Cantonese.
Some movies like Kung Fu Panda or Juno also have Cantonese subtitles, and some older HK movies do too. Song Lyrics from bands like Wynners, ToNick, HardPack, Ping Pung, MastaMic, LMF, etc. are also often very colloquial, so you can read along with the singing :)
Written Cantonese is pretty common on the internet too. Wikipedia has a Cantonese version (http://zh-yue.wikipedia.org/wiki/%E9%A0%AD%E7%89%88
), although many of the articles are very short. Hong Kong based forums like http://bbs.cantonese.asia/
have a lot of written Cantonese, granted the writing may not be "correct".
Greenwood press has a few books in written Cantonese, and as described in this thread (http://www.cantonese.sheik.co.uk/phorum/read.php?1,110693
) there are some comics and other writings that use Cantonese too.
But none of these resources "teach" you Cantonese, of course. They are just supplements or aids. We need websites like Popup Cantonese for the actual instruction and explanations.
If you want to talk about rare written materials, try studying Hokkien :P