According to them, they handled series this way: "In general, works that tell a more or less continuous story are listed collectively (e.g., "The Song of Ice and Fire"). In cases where connections among series members are looser, we tended to list some of the more prominent titles in the run (e.g., Small Gods, a "Discworld" novel)." Which more or less makes sense, looking at the books listed, though I'm a little surprised that "Going Postal" was the #2 Discworld novel (not that I don't love it to death, but I would have expected, say, "Night Watch").

Urban fantasy is not excluded by the rules, but "horror" and "paranormal romance" are, and I'm wondering if some really elastic definition of one of them disqualified The Dresden Files. Because, yeah, there's no way Codex Alera got on the list and DF didn't unless it was specifically excluded.