The WebLogic Server documentation set has undergone a nice facelift in the 12c (12.1.2) release.
I find the breakdown sections generally quite useful to quickly locate the information I'm looking for, but I still regularly go directly to a specific book as a reference.
The entire book set is available here:
I also think the provision of the new mobi and epub formats is incredibly useful, allowing you to pull down a set of books onto a mobile device. I flew to Melbourne yesterday and was able to read over a set of docs on the plane that I'd pulled onto my iPad before I left.
I've been wondering if it'd be possible to provide some sort of library/multi-book selection option to allow multiple book selections to be made at once and have them downloaded as a library that can be pushed to a mobile device? I've not looked into it, but I think it'd be a nice usability feature. Does iBooks have any APIs that could support that? Will need to look int it at some point.