Found a novel in a second hand BookShop the other day called Mysterious Planet which sounds much like the basis of most Niburu stories but this piece of Science Fiction was written in 1953.

[link to www.goodreads.com]

A charming science fiction tale written in 1953 in the middle of the Cold War. Mankind has populated the solar system. Our outpost on Neptune discovers a planet beyond Pluto moving our way. Planet X does not seem to have a regular orbit and seems to be heading toward Earth.