Nearly everything most readers know about Norse mythology comes from manuscripts produced in Iceland in the thirteenth century, roughly two hundred years after Iceland became Christian. Snorri Sturluson, who assembled the Prose Edda, was a Christian writing for poets who needed to understand the old references, and he says as much. The poems of the Poetic Edda are anonymous and undated, preserved in a single main manuscript of the same period.
John Lindow's guide is careful throughout to distinguish what a source says from when and by whom it was written, and the distinction changes how the material reads. A myth recorded by a Christian antiquarian may preserve pre Christian tradition faithfully, may be shaped by the framework the writer brought to it, or may be partly his own construction, and often there is no way to tell which. That does not make the Eddas worthless as evidence, and no serious scholarship treats them that way. It does mean that the Viking Age itself, the period when the religion was actually practised, is known largely through the writing of the religion that replaced it, and that the conversion of the Scandinavian kingdoms is therefore both the end of the period and the condition of our knowing anything about it.