Library permissions are given on the content itself rather than on a screen, so somebody can be trusted with one book and see nothing else. What you grant on an item is carried by everything underneath it.
Open Library, then Browse, and find the series, book, unit or lesson.
Press the shield on its row - Manage Permissions.
Only a full-access admin sees it.
Tick View, Add, Edit or Delete beside the Editor.
Each box is saved as you tick it. Admin and Root always have everything, so they are not listed.
Press Close.
If it says "No Editor accounts yet", make the account first.
Editors are made under Admin Users, reached from the menu on your own name.
Grant at the highest item you can. One grant on a series covers every book, unit and lesson inside it, now and later.
Delete covers everything underneath as well. Give it only where you mean somebody to be able to remove the lot.