Jólabókaflóð, or Christmas Book Flood, is the charming Icelandic tradition of giving books as gifts, opening them on Christmas Eve, and settling in to read them together, ideally with hot chocolate or other warming wintry drink.
The "flood" refers to the traditional release of books in the months leading up to Christmas. While there may be a book flood without gifting them on Christmas eve, it's such a nice extension that it seems worthy of its adapted meaning.
Iceland has a long literary tradition, and a high percentage of Icelanders both read regularly and become authors themselves. Jólabókaflóð has its roots in various causes to do with wars, rationing, ancient sagas, and, I'd guess, also long winter nights.