Tolkien did it, CS Lewis did it and various other authors do it, too.
In the back of their books they add all these interesting little tidbits to draw readers even deeper into their imaginary worlds. Things like a glossary, which lists a brief biography of all their characters; detailed maps of their fantasy worlds and, at times, even a genealogy.
I love genealogies.
Books series' often go through a range of characters, sometimes over generations, so seeing them all presented on one chart is a great reminder of their relationship to each other as the stories progress.
I created one for my Dantonville Legacy series and it was included as part of the back matter in the first paperback edition of Book 1, BloodGifted, but it was harder to do in the digital version, and so it didn't go in.
But, here it is now and I've included an updated version with characters from the 2nd and 3rd books.