My mom was in the hospital and had died.. but when she came back she said that "a spirit told her that se had to go back it was not her time"

Lots of stories, but it is nice to feel such things. It is much more believable that it was her subconscious that was fighting for life, telling her not to give up.

Best book on "spirits" the Bible.

Also you can look up near death experiences. There are many accounts just like what your mother has attested to.