If you are setting up an explorer to find adventures, invest in it.
If you are setting up an explorer for treasure searches, skip it.
Eventually one Codex will cost you 15 manuscripts and I feel it is a waste to use them on something you only use for a few weeks a year.
That said, from my experience it is not true. I have received double event items on explorers without that skill.
One army. Pretty please.
I have seen no correlation between any of the books and double event items. Who really knows how things work? It is super secret. Again I would not plan your life around the events.
One army. Pretty please.