1.固定句型：sb have trouble in doing sth.某人做某事有困难.
该句时态为现在进行时,其中含义一个以who引导的宾语从句.
2.结构分析：
1）主句分析：
主语 We
谓语 are having
宾语 trouble
状语（介宾短语）in finding out who has taken the book (其中finding out后面是其宾语从句)
2）宾语从句分析：
主语 who
谓语 has taken
宾语 the book