What kept Eleanor out of heaven for two centuries?
This book is centered around a version of Eleanor of Aquitaine in the afterlife, and people who knew her while she lived. In real life, in the 12th century, she was first the Duchess of Aquitaine and then the Queen of both France and England.
She was first married to the young Louis Capet, who soon became King of France, following his father's death. After a few years of struggling being married to a ruler who did not know what he was doing, Eleanor requested and was granted an annulment from her marriage (because they were technically blood-related).
Very soon after this, Eleanor married Henry, Count of Anjou and Duke of Normandy. After a couple years, these two were crowned Queen and King of England. If nothing else, you may have heard of their son, Richard the Lionheart.
But I do not need to tell you her whole story—that is why you are reading the book!
To get to the main point of your question, Eleanor is waiting, as she has been for centuries, for her husband King Henry II to finally be let into heaven. In the world of the book, people have to spend time "Below" before they can come "Up," in order to atone for their sins and work on their character flaws. Eleanor also had to wait to get into heaven; her main issue was that she was extremely impatient and that "she had done things on Earth for which there had been some Hell to pay." Because she had been a supporter of the arts—and artists have an effect on the world as well as heaven—she only had to wait two centuries, while Henry had to wait for 8 centuries.