The expansiveness of this library's collection demonstrates its wide coverage of knowledge.

expansiveness 是由形容词 expansive（广阔的、扩展的）加上名词后缀 -ness 构成的名词，表示"广阔性、扩展性"。 wide coverage 中，wide 是形容词，意思是"广泛的"；coverage 是名词，来自动词 cover（覆盖），意思是"覆盖范围"。wide coverage 合起来表示"广泛的覆盖范围"。