word usage - Should the noun be in singular or plural form after "some"?

I usually use the plural form after "some."

Example:

I bought some books about France

But I would also say some source states that France is best toured in fall. Is there a specific grammar rule? May I say that in the second sentence I am really thinking at a very specific source, so singular is needed?

Answer

Some has slightly different meanings depending on whether it is used with a singular or plural noun. When you say, "I bought some books," you mean that you bought multiple books, but not how many were purchased.

With a singular noun, some is used to talk of something whose existence is known, but whose nature or identity is not.

She is in some kind of trouble.
There must be some mistake.