How many letters does the top answer for “a book with paper covers” have?

The most common answer is paperback (9 letters).

Crossword clue: a book with paper covers

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

paperback 9 letters wn-def

a book with paper covers

softback 8 letters wn-def

a book with paper covers

a small book usually having a paper cover 50% similar → booklet, brochure