The 318 bishops issued a creed (Symbolum), 20 canons, and a letter to the church of Alexandria.
But the accounts of Eusebius, Socrates, Sozomen, Theodoret, and Rufinus may be considered as very important sources of historical information, as well as some data preserved by St. Athanasius, and a history of the Council of Nicaea written in Greek in the fifth century by Gelasius of Cyzicus.
Other information about the council is available from the church historians, which also detail action taken by the Emperor Constantine to enforce uniformity after the council. (The works of many of the Nicene and Post-Nicene Fathers are available online
Theodoret, Historia Ecclesia, Book I, ch.6-13. This mentions that the definitions of Nicaea were drawn up with reference to Scripture; and the argument about whether phrase x or y was or was not in scripture formed the basis of much of the argument.
Socrates, Historia Ecclesia, Book I, ch.8. This mentions that Constantine exiled Arius and some of his supporters for refusing to submit to the decisions of the council. It also quotes an letter by Constantine ordering the destruction of all works composed by Arius on pain of death to any found holding them, and referring to a similar past order regarding the works of Porphyry.
Sozomen, Historia Ecclesia, Book 1, ch.21. This describes the results of the council. (Chapter 17 onwards describes the council). Constantine writes to all the cities ordering the destruction of the works of Arius and his followers, and the penalty of death for any who refused to destroy them. The letter is not quoted. There is also an anecdote where a Novatianist bishop is interviewed by the emperor. The bishop agrees to sign the creed but not to resume communion with the Catholics. Constantine tells him to get a ladder and ascend into heaven alone, then; but there is no mention of action against the Novatianists.
Eusebius, Vita Constantini, Book III, ch.6ff. This describes the council without mentioning Arius and concentrates on the harmonisation of the date of Easter. Later it gives the text of an edict by Constantine against heretics and schismatics, 'Novatians, Valentinians, Marcionites, Paulians, you who are called Cataphrygians', banning their meetings and confiscating their buildings. Eusebius goes on to say without quoting that a search was also decreed for their books in order to identify the heretics (although no details are given of what happened to the books).