呢句用左as，係唔係錯？

有一個用法叫 such A as B，意即「像 B 這樣的 A」

例如：
such a dog as C 「像 C 這樣的一隻狗」
such books as D「像 D 這些書」

可當作一般名詞用：
There is no such a dog as C 「那兒沒有像 C 這樣的一隻狗」
Such books as D are unacceptable「像 D 這些書是不能接受的。」

而你這句最好加上一個 those／the ones：
They could only read such stories as those／the ones had been rewritten in sample English

2014-02-21 21:33:48 補充：
更正：
They could only read such stories as those／the ones [that/which] had been rewritten in sample English

意識上你說得對
假如講述那些(故事)書是以簡易(我估是寫錯了sample吧)英文寫成的，用that是對的(which並不太正確)
They could only read stories that had been written in simple English.

改了用as是因為作者想表示原故事並不是簡易英文，而由於他們能力低，因此他們只能讀變為(as)簡易英文寫成的版本。即是stories的原版並非written in simple English，他們讀的版本是由原本的英文改write as simple English.

你作句These were my books as had been rewritten in sample English.
句子的意思便會是
這些書都是由我改寫成簡易英文的。

動詞as had been written的主角是前面的my book，而動詞是(改)寫作，因此主角是你改寫作為簡易英文的書。我估計這不是你寫這句的原意吧。

你可能想表達是「我擁有的這些書都是以簡易英文寫成的版本」。
這樣意思用that會較合理，另外由於你的重點不是在改寫(跟上面那句強調他們讀不懂原英文版，所以要特別改寫為簡易英文，重點是書的規格，因此主詞最好改為these books of mind (或these books that belongs to me...)而不是寫成my books；動詞had been written亦要改為passive past tens(意思是fact的事實如此而不是改調"動作")。

These books that belongs to me were written in simple English.

Simply use 'that' in both cases!