How many letters does the top answer for “a collection of books” have?

The most common answer is bibliotheca (11 letters).

Crossword clue: a collection of books

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

bibliotheca 11 letters wn-def

a collection of books

a collection of hymns; a hymn book. 67% similar → hymnal

a book containing a collection of songs 50% similar → songbook

a book containing a collection of stories 50% similar → storybook