Buscar todos los coautores - Facetado/Agrupación para la tabla de mapeo de muchos a muchos

Prueba esto:

SELECT "AuthorId", COUNT(*)
FROM BookAuthorMapping
WHERE "BookId" IN (SELECT "BookId" FROM BookAuthorMapping WHERE "AuthorId" = 1)
GROUP BY "AuthorId"

Demostración aquí

Alternativamente, puede usar un INNER JOIN :

SELECT t1."AuthorId", COUNT(*)
FROM BookAuthorMapping AS t1
INNER JOIN BookAuthorMapping AS t2 ON t1."BookId" = t2."BookId" AND t2."AuthorId" = 1
GROUP BY t1."AuthorId"

Demostración aquí