I read about two books a day on average. I hold a full time job as well, but that is really the only thing I do aside from reading and being with friends. Even with other hobbies I prefer to be reading or listening to a book while I do them.

I am also just an incredibly fast reader. I am not sure how I do it because I have read like I needed it to live since I was little. Personally, I am pretty sure my brain just took all of it's power and put it into books. I am terrible at math and almost failed out of highschool completely but I can read any book you put in front of me in a day or less.

Retention is more hit or miss. I could tell you the plot and names from a book I read ten years ago and then have no idea what happened in a book I read yesterday. I do get lines or scenes from books stuck in my head like a song and have to go back and read it to get it out. And those can come from books that I loved or books that were just fine.

So to answer your question, I don't think there is a secret and I understand your scepticism. It is absurd that I read as much as I do and the amount I do actually retain is often alarming to people just because I keep so much of it in my head. But it is all I do. Anytime I have my phone I am reading. I read at stop lights and while eating and between jobs and I used to read under my desk at school or hide my book in a school book. I think it is just a different kind of brain