当 a stack of + 复数可数名词，谓语动词可单可复，复多见, stacks of +复数可数名词，谓语动词用复数。

例如：

1.Then a stack of books are brought in from the camp library.

2. Is it because a stack of books is facing me?

3. Stacks of books are nothing if we have no idea how to choose among them. 

A stack of + 不可数名词，谓语动词用单数：

If a stack of paper is left standing in air that is too dry for it, the edges will lose moisture and shrink. 

Stacks of + 不可数名词，谓语动词一般用复数,单数极罕见：

1. Stacks of paper are piled upon every surface and across virtually every inch of the floor. 

2. Those stacks of wood is holding you back from your future man.