This question already has an answer here:
I am trying to find the name and author of a children's fantasy/sf book. The plot involves people who are simply disappearing from the world. Usually they come to some decision and mysteriously tell their family they are leaving, and soon after they simply cannot be found.
The protagonist (I think a boy) discovers that the family cat can see strange black blobby creatures that are invisible to everyone else. (This is why cats stare at things we cannot see.) I think the cat decides to "leave" as well. Finally, the boy discovers that the blob creatures are the connection to another world where everything is rural. He and his family step into the other world. The story ends with a description of how they farm the blob things.
I think the author is female.