How other biographers writing in circumstances similar to those of the gospel authors viewed their work:
As Aune notes, "while biography tended to emphasize encomium, or the one-sided praise of the subject, it was still firmly rooted in historical fact rather than literary fiction." Ancient historians such as Tacitus thus felt free to draw on biographies as well as annals and memoirs....
Even in biographies, most writers felt free to record negative as well as positive features of their protagonists, when appropriate. Thus, for example, while Suetonius' Vespasian is mostly adulatory (a striking contrast to his biographies of Caligula, Nero or Domitian), he reports his pecuniae cupiditas, love of money....Plutarch, too, knows how to criticize even his favorite figures at times....
In both cases [works written on the Roman emperors Galba and Otho within a few decades of their death], synoptic comparisons among such works reveal that the key biographers viewed their role as adapters of historical information rather than as free inventors of stories. (16-7, 38)
One of the points that's made often in the book is that the gospels are closer in time to their subject (Jesus) than most ancient biographies were:
The Gospels may reflect a more popular level of writing than most extant biographies by members of the elite, but (in favor of more substantial information in them) they are also closer in time to their subject than are most ancient biographies....
Other written sources, probably noted in Luke 1:1, may have been circulating in the same [pre-gospel and early gospel] period. We do not know when the first gospel sources were written, but a finished Gospel [Mark] was completed within a generation [of Jesus], at the latest. By comparison, most of what we know of other ancient history comes from historical and biographic sources written far more than four decades after the events that they narrate. (2, 331)
It's popular today to criticize the gospels for not citing more sources. I've addressed that objection before, and anybody who's interested can read that post I just linked. The book I'm currently discussing addresses the subject to some extent. See, for example, pages 22-3, 84-5, 105, and 139. I'll have more to say about this issue later, when I discuss some of the book's shortcomings.
One reason why biographers (and others) often didn't cite sources or include other types of information in their accounts was that they considered the material to be common knowledge in their day (as we today often omit or don't document material we consider common knowledge):
Tacitus omits reporting most of Seneca's dying words, recorded by the latter's secretaries, simply because in Tacitus' day they remained too well-known to merit repetition in his work (Ann. 15.63)....
In Alc. 7.3, the writer [Cornelius Nepos] mentions his own conviction regarding why something occurred without supplying a source. It is noticeable, then, that he feels no need to defend his authority on the material presented, nor does he sense a challenge from his audience to defend his view of the events. (24, 83)
There are a lot of places in the book where they discuss how non-Christian sources often have the same sort of alleged inconsistencies that critics object to in the gospels. For example:
One unsurprising conclusion in the study of ancient biography is that such differences [as we find among the gospels] are not unusual in ancient biography....
Accounts in Suetonius, Plutarch and the historian Tacitus correspond with one another in ways similar to how the Synoptics correspond. This is significant because Suetonius and Plutarch are the key extant examples of biographers from the early Empire. (2, 143)
Entire chapters in the book document in depth what Keener is referring to in his comments above.
For an example of two non-Christian biographers differing in wording, what they include and don't include, etc., even when they seem to be relying on the same source, see pages 210-2.
Many of the characteristics we see in ancient biographies can be found in modern sources as well. Keener provides some modern examples of how differing accounts can and should be reconciled. Often, further investigation demonstrates that there's more consistency than there first seemed to be among differing accounts that we come across in our own day. Keener mentions some examples he's had firsthand experience with (n. 53 on 164).
(Upcoming segments in the series will be linked here when they become available: part 3, part 4, part 5.)