Question: Among public magnet schools,what percentage of schools that receive the donated resources as books?
SQL: SELECT CAST(SUM(CASE WHEN T1.project_resource_type = 'Books' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.projectid) FROM resources AS T1 INNER JOIN projects AS T2 ON T1.projectid = T2.projectid WHERE T2.school_magnet = 't'