Q. Does CMOS prefer “best seller” and “best-selling” per the dictionary spelling (over AP style of one word, no hyphen, for both)?
A. Merriam-Webster updated its entry sometime in 2022 to list “bestseller” (one word), followed by “best seller” (two words) as an equal variant; meanwhile, the adjective “best-selling” remains hyphenated (as of August 2026). We normally prefer first-listed spellings (even for equal variants), so bestseller and best-selling would now be our preferences. That said, an editor might be forgiven for preferring bestselling as a matter of consistency with the noun form, despite what M-W says.