I mean,a student of English is distinguished from what is called the general reader.what不能用who

可以,但 who 前面要加 one 或 someone,并这样改：
I mean,a student of English is distinguished from one who is a general reader.
（虽然原句无错,本人比较喜欢第二个说法.）
作者用 what 而不用 who (one who is …) 是因为他把 “a student of English” 这个短句当作物体 （即一种名称）来用.
如果以 it 来代替 a student of English 这就很容易明白了：
It ( “a student of English” ) is distinguished from what is called the general reader.