Hi Toni! I'm not a native Cantonese speaker, but I wanted to offer my thoughts because I've tried to look into this question too. Alas, as far as I can tell, almost all books and stories (even most children's books) are written in standard Chinese. They can be read with Cantonese pronunciation, but the vocabulary, grammar, and word usage will be standard (i.e. you'll see 你們 instead of 你哋, 他 instead of 佢, 沒有 instead of 冇, etc.). It's very hard to find texts in colloquial Cantonese.
The same is true for subtitles in Cantonese-language movies. Occasionally you can find a movie that comes with verbatim, colloquial Cantonese subtitles, but they seem to be very rare.
I assume the same is true for poetry. Like song lyrics, some poetry may be written "in Cantonese" in the sense that the sounds of the words flow better in Cantonese than they do in Mandarin, but the grammar and vocabulary will probably still be standard Chinese. *Some* modern song lyrics (I think mostly hip-hop songs?) are written in colloquial Cantonese, I've been told, so maybe there is also some modern poetry of this kind, too. But in my experience it seems like most Cantonese song lyrics are still written in standard Chinese, and I'm guessing this is probably even more true for poetry.
I've been told that a good place to look for colloquial Cantonese writing is in 漫畫/comics and graphic novels, including online comics. I've been told that you can find Cantonese comics on Instagram -- I haven't really looked into this yet myself, but maybe someone else here will have some suggestions!