• The Annals of St. Bertin is the most important source for the activity of the Vikings as well as more general politics in Western Frankish kingdom, including their attacks on Paris, in the 9th century.
• The text itself of the alleged treaty of Saint-Clair-sur-Epte in 911 that granted Rollo the land around Rouen is now lost, and only alluded in the most important, but difficult text in early history of Norse Normandy, History of the Normans that was written in 1010s. Some parts of this work by Dudo is available here on the internet.
• At least one documentary evidence (charter) nearly contemporary to the lost treaty, issued in 918 to the abbey of Saint-Germain-des-Prés, also mentions Rollo and the treaty, and the original charter is extant. The following is an excerpt of the translated passage in question: '… we give and grant this abbey of which the main part lies in the area [pagus] of Méresais on the River Eure to Saint-Germain and to his monks for their upkeep, except that part of the abbey [‘s lands] which we have granted to the Normans of the Seine, namely to Rollo and his companions [comitibus], for the defence of the kingdom [pro tutela regni] …' (van Houts ed. 2000, no. 3: 25:).
• The annals of Flodoard of Reims (919-966) also sporadically mentions Rollo and the Norsemen settled in Normandy (especially Rollo's son, William Longsword).

Unfortunately, none of them in full form are available free in English and on the internet, but the excerpt of relevant documents as well as that of lengthy account by Dudo (especially Rollo's part) is conveniently translated and collected in Elisabeth van Houts (ed.), The Normans in Europe, so I recommend you to check this primary text collection, in addition to Dudo's online partial translation (see the link above).