How many letters does the top answer for “a reference book that is published regularly once every year” have?

The most common answer is annual (6 letters).

Crossword clue: a reference book that is published regularly once every year

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

annual 6 letters wn-def

completing its life cycle within a year

yearbook 8 letters wn-def

a reference book that is published regularly once every year

yearly 6 letters wn-def

a reference book that is published regularly once every year