Those stacks are by author's last name (I've never organized my library so clinically before, and I used to oppose such rigorous shelving rules, preferring instead to place the books where they felt right, but I guess I'm finally an adult because alphabetizing is nothing short of thrilling to me now). According to this actual, physical bar graph, the books I own the most of are by authors with last names beginning with "M" (we pretty much have the complete works of Melina Marchetta to thank for that). The least populous letter is "Z" of which there is only one author inhabitant. Think you know who that is? Highlight here to see if you're correct: Markus Zusak.
The things you learn when you alphabetize!