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To be "in the same timeline" typically will not mean anything except that the characters experience the same "watershed events" that basically everybody in their world hears about. But such watershed events are rare. Examples might be the death of a ruler, a war begins or ends, a devastating flood or hurricane or tornado, a scandal, an important murder or conviction, a fad or new invention or new 'celebrity' becoming known, a new religious leader emerges. A country is conquered, a revolution succeeds or is decisively put down, or a country is officially split (as Iraq may someday be). Something iconic is destroyed (a fire takes an ancient forest, our own 9/11 destruction of the World Trade Center, a famous church or landmark is bombed, or destroyed in an earthquake.
Of course they will also likely have the same technologies at their disposal, or close to it with minor variations, but that depends on your story line, I suppose you can have flint knives versus laser guns if you want, but in that scenario it might be more difficult for the flint knife crowd to be aware of the same watershed events as the laser gun crowd.
Since you contemplate ten or more books, you should have the imagination to come up with a book-universe wide history of what goes on in this timeline.
The watershed events do not necessarily have to be plot points, just markers: In every book, early on, somebody mentions an assassination, in gossip:
"Doctor Havil is dead, did you hear? Assassinated, they say."
In each book with various responses: Shock, joy or celebration, disdain, whatever. Pleasure: "Really? Then Arthur owes me a beer, I told him Havil wouldn't last the year!"
But that can be the full impact of Doctor Havil's assassination, to the plot or book. It doesn't have to really matter, just as it doesn't really matter to us today if some astronaut should die in space or some Hollywood icon gets convicted of rape. We hear about it, but our life goes on, unchanged.
You use other watershed moments similarly; but sparingly: The moment I mention Doctor Havil's assassination, my timeline is synchronized, in every book. That is the same moment in time for everybody, Doctor Havil can only be assassinated once. The same could be said for an earthquake in California with a particular, unique consequence:
"I just heard the earthquake yesterday collapsed the cave of Pilotes tomb. I can't believe it."
"How is that possible? That cave has been there forever! Did it have pilgrims in it?"
"I have no idea, my wife heard it from a friend."
Once again, timeline synced, every time this is mentioned. Once near the beginning of the book, and another watershed event near the end (related for 'closure' if you want; e.g. 'they' schedule a reopening of Pilotes tomb).
If subsequent trilogies move forward on the timeline, your 'closing' watershed event in other books or trilogies becomes something that recently happened in your new book or trilogy.
Just make sure your watersheds have some detail in them so they are unique and cannot possibly (or at least plausibly) happen again. JFK was only assassinated once, Pearl Harbor was only ambushed once, it is implausible that would happen again. The World Trade Center will never be destroyed by terrorists again. There will not be another American Declaration of Independence, or Emancipation Proclamation, or end of Prohibition. There will not be another day in which same-sex marriage became legal throughout the USA (06/23/2015).
If you need ideas of such events, I'd suggest looking at three or four big newspapers, online, say from New York, Miami, Los Angeles, Houston (whatever you can find for free). Or websites, like Huffington, Fox, major networks, CNN: And find days when all of them have a front page headline about the same news item. They don't have to agree, their spins may be different, but that gives you a clue to what kinds events are watersheds and iconic. The day after the election of Barack Obama, or the day after Katrina, no news outlet failed to have a prominent story on the event.
I do say use them sparingly; unless your story makes great leaps in time, one watershed early on is probably enough. If it does make great leaps in time, have one watershed event for each major time segment. A simple quarter-page conversation in each book will not be intrusive and will anchor the reader. but make the conversation a little different in each book, based on the personalities of those discussing it; their reactions can be different, or even if they all express regret or all express elation, they have their own idiosyncratic ways of doing that. If you choose the common event carefully, these reactions can double as character revealing moments.