For the last decade or so I have always read two books at the same time. The practice began when I was still studying, because it helped me save time when I studied an easy subject like English Literature with a tougher one like Calculus. I would start out with the difficult one and when I could feel the mind closing and refusing to take in any new concepts, I would switch to the easier one.
Helped to keep me mentally agile and able to fill in a lot more stuff in the same time frame. Then exams became a thing of the past and I was free to pick and choose whatever I wanted to read. So I would again pick classics from Milton or Shakespeare and couple them with an Agatha Christie or a Sidney Sheldon. I also liked to read all the books of one author at a trot. The "Complete Works" series must have been made to order for me.
I love reading Omnibuses. Yes, I delight in going through all the stories with the same characters one after the other. Specially if the characters are Jeeves and Bertie. The fact that I sat down and read the full Jeeves series in the week before my tenth class boards, and still managed to score ninety out of hundred in English is one of my favourites.
I speed read and so don't usually get much beyond the basic plot and characterization in the first reading. In fact if I were to pick up a book I just finished reading today in a year's time, chances are that I would only realise that I have read it before after I am half way through, and then I will have to read it again as I most likely have forgotten the end.
The reason I am harping on and on about this reading habit of mine is that it is this one hobby that has helped me develop a great vocabulary. The more I read the better it gets. And given my current line of work, that is what is needed more than any other skill.