How many letters does the top answer for “another word for textbook” have?

The most common answer is casebook (8 letters).

Crossword clue: another word for textbook

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

casebook 8 letters gen-syn

a book in which detailed written records of a case are kept and which are a source of information for subsequent work

schoolbook 10 letters gen-syn

a book prepared for use in schools or colleges

another word for about 50% similar → approximately, roughly, some

another word for terms 50% similar → footing

another word for under 50% similar → nether

another word for word 50% similar → phrase