It's a weird metric because a "book" is such a variable thing. Last year I read 27, but the last 3 that I have read this year total 6500 pages alone. You can read a romance novel in an afternoon, but my last Joe Abercrombie book took like 32 hours, and 14 books of wheel of time took me 3 years.
All that to say, I feel like there should be another metric, but non-space characters seems the most fair and also a bridge too far :).