The Reign of King Viserys, first of His Name, and the Dance of the Dragons That Came After
When Archmaester Gyldayn wrote The Princess and The Queen, or, The Blacks and The Greens, he used Eustace's writings as one of his sources because they contained information not found elsewhere. However, Gyldayn also felt that Eustace was a green supporter and biased to portray Aegon II in a positive light and Rhaenyra in a negative light. Therefore, Gyldayn was often careful to weigh statements from Eustace's writings with other quotes from contradicting sources, hoping that the truth could be discerned somewhere in between.
Unfortunately, only fragmented copies of Gyldayn's works survive, often poorly edited. The result is that certain points in which Gyldayn was careful to state that he was quoting something Eustace wrote have been omitted in later copies - the result being that statements from Eustace's work are presented as if they were statements of fact by Gyldayn.
George R. R. Martin has explained that the printed version of his novella The Princess and the Queen was shortened for length by his editors, a result of which is that sentences which he originally prefaced by saying that they were based on Eustace's account were cut, presenting this information as fact. For example, the point in The Princess and the Queen when it is stated that Aegon II did not want to be king (but was goaded into it by his mother) is presented as simply a statement of fact by Gyldayn. In Martin's full draft, however, it is clarified that this is the version recorded in Eustace's book - implying that his account was biased and attempting to make Aegon II appear humble, when this is not really what happened. Aegon II's personality in much of the rest of the narrative describes him as a greedy man grasping for power, shedding further suspicion on Eustace's account.