In an invalidation search its been very difficult to look for non-patent stuff before a particular year, lets say before 1994. We have already discussed few strategies, such as Google timeline etc., but as I always say – New strategies and sources should be constantly evolved.
Another very useful method for non-patent literature in patent invalidations search could be information about the History of a particular technology available in Wikipedia.
For example, if you are invalidating some patent that relates to Virtual Learning, then a good way to start would be to look here:
Though Wikipedia cannot be used as a reliable source but it provides enough hints that may help the researcher to reach to an authentic page.