Official documentation for Islandora 6 and 7 is available on the Confluence Wiki.
Official documentation for Islandora 8 is available on GitHub.
Further documentation has been created and shared by members of the Islandora community:
PALS Islandora documentation (Alex Kent)
Batch Uploading Books into Islandora with Drush (Amanda Lehman)
Connecticut Digital Archives Islandora Resources (Jennifer Eustis)
Create Taxonomy for XML Forms (Jennifer Eustis)