摘要：The book isn't here. Where else (= in what other place) should I look?（这本书不在这里。我还应该去哪里找？）[剑桥高级学者辞典]

副词else的含义是“别的、其他的、另外的、不同的”。

who else, what else, how else, why else，where else 都可以，如

【例句1】The book isn't here. Where else (= in what other place) should I look?（这本书不在这里。我还应该去哪里找？）[剑桥高级学者辞典]

【例句2】He came to see you. Why else (= for what other reason) would he come? （他来看你。他又为什么会来？） [[剑桥高级学者辞典]

【例句3】After I'd thanked them I didn't know what else (= what other things) to say. （我谢了他们之后，不知道还能说什么。）[剑桥高级学者辞典]

【例句4】Who else was at the party? （聚会上还有谁？） [朗文当代英语词典]

但是whose else不可以，可能是因为whose是形容词性的。

which else也不可以，估计也是因为which容易被解读为形容词性。

反正《剑桥英语词典》的语法是这么建议的。

来源：语堂教育