I know some people read one book at a time cover to cover as they want to keep the same train of thought. I read multiple books at the same time, I feel that reading books at the same time in the same or different areas allows for a crossflow of ideas and then possibly germination of new ideas triggered by reading two or three books at the same time.
For remembering those ideas I usually either bend the corner to quickly reread when I have finished the book. I also take notes in my phone with title, page number and idea I had, this allows me to come back through the reread and see if there is something I wish to action on or research further.
I try to read 12-20 books a year along with the numerous blog articles and academic papers. Most of the books I read are non-fiction, I will read fiction every so often but usually on holidays when I want to try and take a break from work, WLA and other business pursuits.