How many letters does the top answer for “which is not a book in the harry potter series” have?

The most common answer is elf (3 letters).

Crossword clue: which is not a book in the harry potter series

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

elf 3 letters otdb

below 3 kilohertz

house 5 letters otdb

a dwelling that serves as living quarters for one or more families