When I started the sketches that would eventually coalesce as The Books Of Loukas 5 years ago, I had no idea what I was going to have to actually do, in terms of detail work.
Along the way, apart from inventing 20 or so characters, I realized that I needed to invent an entire town, geographical area and other aspects of an imaginary world.
This is not like Tolkien, who invented not only a complete world but also its own languages (albeit derived from Old Norse, a language in which he was one of THE world experts for decades). My world is based on real areas, but the towns, people and other aspects of geography are imaginary.
I have a lot of material about this world. Maps, descriptions, summaries.
There a number of options for how to address the issue:
1. Simply put all of the content in the main line of the books
2. Put the content in Appendices in the book and set up internal links in the eBooks
3. Put the location in a different ebook on the same reading platform and set up an external link to and from the eBook
3. Put the content in a different location and link to the location from the eBooks
(1) will increase the size of the eBooks, and there will be repetition across books
(2) will also increase the size of the eBook, but will reduce peripheral diversions from the main line of the story
(3) will minimize the size of the eBook, but the reader will need the reference eBook in the same location as the eBook for the links to work
(4) will minimize the size of the ebook, but will prevent offline reading if there is no active internet connection
A complicating factor is that the writing software I use (Scrivener) does not support some of the linking approaches listed above, so I would have to perform post-processing on the eBook files to make them operate in modes (3) or (4). Additionally, I think that linking to a location within an eBook on a web site will be difficult. It would be better to link to a more conventional page within a page structure.
So, I will be experimenting in the next few days. In the meantime, I will assemble a Background ebook anyway. You never know how well-sorted something is until you try to make it into a publishable artifact.