When historians talk about the present they are referring to events that are happening at this time. They must not precisely talk about events happening at the same time, they could take five years to refer to as the present.
If you´re reading a book and in there the author talks about the present you have to place yourself in the context in which the book was wrote. For example, a book of 1920 talking about the greatest war the world has ever seen he is referring to World War I.
