核心问题
...even my uncle's huge collection of books ____ (was/were) kept miraculously free from dust.
……甚至姨夫的大量藏书也是奇迹般地一尘不染。
按我的理解 a collection of books 在意义上相当于 a number of, 因此其谓语动词应当用复数 were。
词典解答
应当记住,语言现象常常不能靠逻辑推理的方法来解决,而是应当从惯用法中去寻求答案。按照英语惯用法,a collection of + 复数名词的谓语动词必须用单数:
There is a collection of pictures at the Town Hall. (JED) 市政厅收藏着一批画。
A large collection of contemporary photographs, including some taken by Henry, are on display A B C D
at the museum. (TOEFL)