I often have several books on the go. Right now i am reading the "Life of Pi" , "Harold:The Last Anglo Saxon King" and "Bede's Hitoria Ecclesiastica"( though that last one is actually a printed out copy of translation I found online
I have great difficulty with crossover if the books I am reading are too similar though.....so....If I am reading fiction ( seemingly rare these last couple years ) I might have a humourous contemporary novel on the go at the same time as some high fantasy....Usually always have at least one piece of historical analysis on the go....I found though that I had to give up reading any historical fiction set in the middle ages when I have historical research/study to do as that tends to confuse matters.....