Put your series in a universe
Why a book has to belong somewhere
A codex lives in a universe, and a book reads the codex through its series: if the series is in the universe, every book in it sees the codex, and every entry can say "this is true in book 3" by name. Until a series is in a universe, its books have no codex to read—the Codex rail in the editor will tell you so, and so will the codex's own page.
So the order is: make a universe, put the series in it, open the codex.
Make a universe
From the Library, click Codexes in the Manage row. You'll see the Universes page: every universe you have, under its pen name, with an Open codex button beside each.
Click New universe, type its name (Aeon 14, say), choose which pen name it belongs to if you have more than one, and click Add. That's the whole of it—a new universe comes with a Gregorian calendar and the built-in entry types, ready to go.
You can also make one from Pen names, where each pen name lists its universes.
Put a series in it
Back in the Library, look at the series heading. If the series isn't in a universe yet and its pen name has one, you'll see a dashed button there—+ Aeon 14, or whatever you called it. Click it and the series joins the universe; the button becomes a small tag with the universe's name, and clicking that tag opens the codex.
If a pen name has several universes, the … menu on the series heading lets you choose which one. A series can be in one universe at a time, and moving it is as easy as joining it.
A standalone book
A book that isn't in a series can join a universe directly, from its Detailsdirectly: in the editor.editor, open Book → Details and choose the universe under Imprint. A book in a series always takes the series' universe, so there's nothing to set on the book itself.
When the codex is in the way
Any book can switchstop reading its codex off from Book → Details—codex—say, a book in the series that's really a side story inset anothersomewhere world.else. In Book → Details, untick Reads the codex. It keeps its place in the series; it just stops reading the codex, and the Codex railrail, saysthe so.underlines, and anything an AI sees through that book go quiet until you tick it again.
When a universe grows
New series join the same way, whenever they're made or later. One universe can hold as many series as the world does, under one pen name. If you write two unrelated worlds under one name, make two universes; if you write one world under two names, that's one universe under one of the names, and it's worth thinking for a moment about which.