My system isn't anything exciting, but I find simple works best for me. In my Notes app I have a note titled "Books to read". I have it set up with check boxes and wheneveyI come across a title that looks interesting I add it to the list. Once I've read the book I check it off. If it is a keeper, I add the book to a different list I have in Notes called "Good Books". I know; not flashy, but works for me.