Where can I learn twitch language?

The obvious answer is to just watch Twitch. A lot of the jargon they use in their livestreams is new to native speakers as well. I would be very surprised if there are any books about it.

Cant imagine anyone from twitch audience/streamers writing a book or an article about wildly different slang. Anyone outside of those 2 categories is unlikely to be educated enough to truthfully write about it without really diving in essentially becoming either viewer or a streamer. Also a pretty big portion of slang can be very local and only relatable/understood by 2-3 streamers and their shared audience. Most of the rest is just modern internet slang and you would need to give specific examples that you want explained. It may not be twitch specific and can be gamer slang or content-creator slang or something else entirely. Also you may be confused by people spamming some short messages and those might be emotes like KEKW and such. Make an effort and install some addons for your browser like 7tv, bettertv, ffz, etc, so you can actually see those emotes.

[Urban Dictionary](https://www.urbandictionary.com/) is probably your best bet. But a few disclaimers, it is open sourced so not the most reliable, it's always good to check the definition provided does in fact work in context. It is incredibly nsfw. Do not go digging around expecting to find just fun slang, you will stumble across something incredibly offensive eventually. Otherwise you just have to wait for the OED to start including Twitch slang, but considering they haven't even added the collective pronoun definition of "chat", I think you might be waiting a while.

All ingroups reliably create jargon, phrases, terminology and slang that is really only understood by the ingroup. Many of the words we use today and are deemed "common" were once slang used by a small group that gained popularity. Much of English invention in America has recently risen out of queer PoC spaces, such as "killing it", and then followed a bottom up pattern. https://www.columbiaspectator.com/arts-and-culture/2022/09/29/new-uses-old-words-how-black-lgbtq-culture-influences-modern-internet-slang/ You just need to spend time in the culture to learn the specifics of the meanings. It comes with time