What they say in the Asterix books is true. When describing an event at which he was present, he does generally say "Caesar did ..."; though I notice that when relating an event at a distance, he often refers to "our troops" or "our cause". Assuming this is an accurate rendition of the text into English, it's a curious mix. Perhaps any civil war is a confusing situation; the narration, though clear in the detail, doesn't really give a wider picture, though Jane F Gardner's introduction attempts to explain the background at a basic level. It feels as if this text is less propagandistic than The Conquest of Gaul - perhaps because the enemy was composed of Romans rather than barbarians - though there's still enough spin to be notable.
The authors of the continuation texts are unknown; only The Alexandrian War is probably comparable in quality to Caesar's writing. Nevertheless they are useful in providing source reports of the developments in this period of history.