Generally speaking, the Vikings were not a literary society. They didn’t write their histories while making it. Much of the information we have about them comes from those who were pillaged and plundered. A few sources are contemporaneous Christian and Muslim writers. However, much of what we believe we know about them comes from Vikings in writing in the form of sagas which contain skaldic verses.
Christian Sources
The Christian sources available include the Anglo-Saxon Chronicle, Frankish and Irish Annals, the letters of Alcuin, and Wulfstan’s “Sermon of the Wolf.” The latter two interpreted the Viking raids as God’s punishment of the Anglo-Saxons. Because these were written by Christians and the Vikings were attacking monasteries for their wealth, the Vikings were painted in a negative light and their atrocities amplified. Christian monarchs, who participated in similar activities, were not condemned religiously.
The Icelandic Sagas
These stories, written after the academically accepted fall of Viking relevance, were often used to support a king or noble by connecting him and his family with events of the past. At one time, historians accepted these tales as fact, except when they were clearly using mythology. The pendulum swung the other way, and historians decided that the sagas were worthless as historical documents. Currently, historians recognize that the sagas have more than just literary value; they just have to be used carefully when looking for historical value.
Skaldic Verses
Many of the Skaldic verses that have survived are written in the sagas and attributed to a known skald (or poet). These verses were written at the time of the event, and the form used was complicated and structured. This form actually lends credibility to the skaldic verse. While the story was passed down orally until its writing in the sagas, its use of alliteration and meter would make it difficult to forget or change without disrupting the structure.
Still, skaldic verses may have been created at a later date and attributed to the original skalds. These verses also contain highly poetic language that can make them difficult to understand. Finally, the strict structure also meant that skalds were more concerned with verses that worked. The accuracy of the historic event presented in the verse was somewhat less important.
Other Sources
The Vikings had their own form of writing: runes. These symbols were usually carved into stone, and so their messages are short and often give only little information. Minted coins also provide some information about Vikings. Most notably, the dates on coins in graves can guide archeologists to establishing the year the dead were interred.
Sources: https://www.bbc.co.uk/history/ancient/vikings/evidence_01.shtml
