Does anyone have any pointers on how to make this work?
I have a 100% superwash cashmere. I frayed the ends and got them wet, then rubbed them in my palms to try and felt them together. I rubbed for probably 5 minutes and they look like they are one strand, but as soon as I put any pressure on either side of the join it just pulls apart. Any tips on what I might be doing wrong?
Your technique is fine. It’s your yarn choice. Superwash yarn is made to be able to go in the washing machine and sometimes dryer so it won’t felt. You won’t be able to do a felted join.
If you don’t want to weave in ends you can try a Russian Join. Not all yarns want to do that well either so you may have no choice, but to weave in.
Alrighty then. I didn’t realize that would be difficult. I thought that you could felt anything that was of animal original. Well learned though. I suppose I’ll suck it up and weave.
That’s a common mistake. At least you were only trying to do a felted join and hadn’t made an entire project that was supposed to felt! 
So. Since obviously if I’m gonna be better at this I need to learn more about fibers. Any suggested books?
Hmm…there are lots of books, but I don’t know if one is better than another. Here’s a website with info.
Generally, something with high % of animal fibers will felt, except for superwash…
I love the felted join, but I have to say that on one yarn I used, it didn’t look good, so I used another join. Still, my favorite. Second place goes to… russian join! 
You can use it with almost every yarn, except maybe 1-thread yarn.