I was taught that there is always a comma before "which." If a comma changed the intended meaning then we should use "that" instead.
- The resources, which you may find helpful, can be found on the fifth floor of the library.
- The resources that you may find helpful can be found on the fifth floor of the library.
- The resources which you may find helpful can be found on the fifth floor of the library.
So 1 and 2 would be correct, but never 3.
However, I have often seen "which" without a comma. Is this one of the rules that have changed?