Just looking through some of the books I've rated and I've found one that might be good.
It's called Sum: Forty Tales from the Afterlives. It's around 120 pages and it's a very easy read, so could be a good one to get things going. It's essentially 40 very short stories of different types of afterlives. I've always found the concept of afterlife fascinating, so I really liked it. Some reviews on Goodreads found it repetitive, so it could be an interesting one to discuss. I also read it around 5/6 years ago, so I'm curious if my opinion of it would have changed over the years.
I found it in Mobi format: