In general, a book should not be used as source material if it is more than four years old. (An exception to this rule might be historical research). Why is it important to support assertions with relatively current information?
In this information age, especially in scientific areas, new discoveries are made rather frequently. Some of these discoveries may contradict previously held ideas.
(We no longer think that the earth is flat, do we?)
I hope this helps. Thanks for asking.posted by PsyDAG