[EDIT: See the Drupal handbook for the current PDF]
This year has not gone as I had expected for a variety of reasons, but that's all right.
Long term sustainable documentation with Drupal is complex. Drupal core moves and changes in ways that have unpredictable effects on documentation. Modules get absorbed into others, some removed, technologies added, behavior changed. Adding on the additional explosive growth of the contributed modules it's mind boggling. How to structure documentation in a way that people can have defined targets for what 'must' be updated vs random one time contributed document that only applies to a particular configuration?
The three main books for people,(install / customization / developer ) are currently at 1414 printed pages. That's an insane number to maintain. The developer handbook itself is generally kept up to date in the key areas by several developers who understand the benefit of being able to look up something they've figured out.
Customization has theming and it would be nice to break that out to it's own book, but so far the folks who have said they were going to do this have not done so. I have someone working on stuff for Drupal 6 theming so I still have hope for the future there.
So, what is the one thing we need to do better on for a major version release? Installation and configuration.