I can see a reason for both.
I tend to read books in series order. So having it automatically show the next books in the series is great.
But another option should be books being read. I sometimes am reading more than one, so any recent books opened, but not bookmarked to the last page, should be able to be shown.
Actually, I need a combination. The next book in the series AND the two books I last opened but did not read to the end.
