What does the cryptic crossword clue "Book contains three-fifths of tales untold (10)" mean?

The wordplay uses a container (one word inside another). LES (synonym="LES") in NUMBERS (synonym="Book") = NUMBERLESS The answer is NUMBERLESS.

Book contains three-fifths of tales untold (10)

Guardian Cryptic #29570 | 13 Down | 2024-12-19 flower