I started summarizing chapters of The Odyssey, and then wondered if I should be or not. If you'd already read it, after all, you already knew what happened; if you hadn't, I was only giving stuff away.
The possible benefits of summarizing are, so far as I can see:
If you read the chapter but didn't understand some of it, my posting might clear up any difficulty.
If you are wondering if you want to read the book or not, looking at the summaries might help you make up your mind.
I tend to discurse on cultural and contextual issues, so you might get extra information from the summaries even if you've already read the chapter in question.
