well it is again all about interest and what you are reading. if it is top heavy technical stuff, I would prefer to read more than one book to feel lees dense at times. If not, hang on to the one if it is interesting and not look at another till I finish reading
When I start a book, I usually don't put it down till I finish reading it. In my opinion, you need to immerse yourself in each books plot to "feel" the plot. Trying to read multiple books will not do justice!!