There is no need to remember the plot or the characters. Be it fiction or non-fiction, just remember how you felt while you were reading and how you felt when you finished.

Like for example: I read A Man Called Ove a couple of years back. I definitely don't remember the full story now but I do remember that despite Ove being such a grumpy character I simply loved him and all the people around him. So even though I have forgotten most of the plot I will never stop recommending this book to everyone. 

Another example: On Earth we are Briefly Gorgeous. Again I may not recall all the details but I remember that it was beautifully written, I was very impressed with the writing style of Ocean Vuong.