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As with any major writing project, I would suggest beginning with some reading. Some good examples of this kind of trilogy-spanning epics include... Anne Mccaffrey's Dragonriders of Pern series...
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Source: https://writers.stackexchange.com/a/32385 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
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As with any major writing project, I would suggest beginning with some reading. Some good examples of this kind of trilogy-spanning epics include... - [Anne Mccaffrey's Dragonriders of Pern](https://www.goodreads.com/series/49339-pern-chronological-order) series which has more than 20 books and several supporting short stories. The trilogies embedded within this massive collection of fiction all overlapping each other on their shared master timeline. They share many characters and events without blatant discrepancies allowing each trilogy to view the timeline from a different point of view. It is a fun ride, but it is also an education on how to handle scale and complexity right. - [Orson Scott Card's Enderverse](https://en.wikipedia.org/wiki/Ender%27s_Game_(series)) is another great example of epic storytelling involving twin quintets of novels, each augmented with multiple short stories and full-length companion novels. There are currently 14 separate works in the Enververse with 4 more in the works. Again, characters and events overlap over a consistent shared timeline, viewed from different perspectives. Another fun and educational ride. Now that you have digested how it feels to have 34 books recommended all at one time, realize what you are asking of your future readers. Nine to twelve books is a pretty big request. I would strongly suggest that you craft each book into a stand-alone enjoyable reading experience. Each should be a complete story which could stand on its own if that was the only part of your writing that a particular reader ever read. In both of the examples above, early novels and short stories were awarded Hugo and Nebula awards on their on merits, long before even their initial trilogies/quintets were completed. Right from the start, they were both excellent! Literature is one area where quantity can never make up for quality. Making each novel stand-alone and excellent has an additional benefit. Dependencies between your stories can limit reader starting points which may limit reader willingness to start reading your work at all. Any book in any of the series above can be your introduction into that series. Order is unimportant because every tale is complete and worth reading on its own. In summary, I would suggest that you focus on what you are writing now and make it great. If you have already imagined the later works and know how they interact with this first story, then plant the seeds of that interaction now, during this initial writing. But stay focused on the work at hand. Let the future works handle themselves.