The continent of Tamriel is divided into nine provinces, the northernmost of which is Skyrim. Its neighbours are High Rock to the west, Hammerfell to the southwest, Cyrodiil to the south, and Morrowind to the east, and it is divided into nine holds: Haafingar, The Reach, Hjaalmarch, The Pale, Winterhold, Whiterun Hold, Falkreath Hold, Eastmarch, and The Rift. Their capital cities are, respectively, Solitude, Markarth, Morthal, Dawnstar, Winterhold, Whiterun, Falkreath, Windhelm, and Riften. Winterhold is the northeasternmost hold, and its capital city is by the northernmost shore.
Each of the holds is led by a jarl, a hereditary position that is passed on to the firstborn child of the previous jarl upon their death or abdication. In the case where a jarl dies and has no living descendants, their spouse becomes jarl if they had one, their oldest living sibling if they don't, their oldest living nibling if they have no living siblings, and so on in a complicated line. Jarls tend to have many children in order to avoid that much confusion when it comes to succession. Jarls do not have absolute power, but given that they are the ones who select the people who have all the power they don't themselves hold this tends to be mostly a technicality.
Furthermore, one of the jarls also holds the title of High King or High Queen of Skyrim, which is also typically a hereditary position. However, unlike jarls, if the High Monarch dies and leaves no living descendants, their spouse or other family relations do not inherit the title; instead, something called a "Moot" is called, in which all of the jarls of Skyrim meet to decide who amongst them will become the next High Monarch.
For the past several generations, the High Monarch has been the jarl of Solitude, and cross-referencing other recent history books reveals the main reason for this: Skyrim is part of the Cyrodilic Empire, and for historical reasons Haafingar is the hold where the Empire holds the most sway in Skyrim.
Map of Skyrim
The most up-to-date book available on recent Skyrim history only chronicles it up to fifty years ago, however, and makes no mention of these "Thalmor", so Ruby will need to consult Urag gro-Shub if he wants to understand that.