Ian has openly admitted (as have many other techinlcal authors) that they can only use information that was known or 'correct' at the time, but, as archives open up and 'new' period documents become available more information comes to light that will either support, or disprove previous knowledge.
The spread of the internet and the freely available information provided by (collectors of the LE) helps increase our knoweldge base, and one obscure document found by a collector in some remote corner of the world would never have become known were it not for the internet & forums such as this.
I remember Peter Laidlersaying (something to the effect)
"you can put all of your years of experience, research & knowledge into a book, but once he has read your book, the collector who has that one little snippet of information you did not find, now knows everything that you do, plus a little bit extra"
There is a reason(s) that books are reprinted with updates.