我认为是填are
白勇语法中提到：what引导主语从句的主谓一致
1）what在从句中充当主语时,从句谓语动词一般与主句谓语动词相一致
2）what在从句中充当宾语时,主句谓语动词由主句谓语动词后的名词决定
（本句中,what是宾语）
what引导的主语从句,谓语动词一般用is.
但是有这样几种例外：
1.如果主句补语是复数名词,则动词用复数：（我们需要的,书应该是主句的补语吧?就是这种情况）
What was real to him were the details of his life.
2.what分句本身明显表示复数意义,比如what分句的动词和补语都是复数,那么主句动词也用复数,例如：
What are often regarded as poisonous fungi are in fact edible.
再比如 what 是限定成分,其后带复数名词,那么主句动词也用复数,例如：
What ideas he has are his wife's.