The question says it all really. If you are writing a WPF application, how are you integrating the application help? What is the state of play in mid-2013?
It seems that there is no clear answer to this from an afternoon with a search engine, but several options:
Write your own fancy tooltip based help (but where are you getting your data from?)
Use .CHM files and the Windows Forms help system (seems archaic to me).
Use Microsoft Help Viewer 1.X or Microsoft Help 2.0.
Use a third-party product to author your help files and link to them somehow - DocToHelp/NetHelp, NetAdvantage on-line help, etc...
Furthermore, what XAML based mark-up / attributes are you using to provide the necessary context? What is the recommended method?
It seems surprising there is no clear path for supporting application based help in WPF.
My current preference is to use a third party help authorizing system to generate HTML based help.
We then use a WebBrowser to display this help as needed. The authoring system we use makes it fairly easy to extract out a single page from the main help (each "topic" is a single HTML file, and can be included with full contents or not as desired).
Granted, this definitely felt like a bit of a nasty hack at first - but once we wrote the basic plumbing (some attached properties for xaml to specify attributes for context location and add behavior to trigger help, etc), it's fairly clean.
One very nice advantage to this approach, however, is a single help system build works perfectly in all contexts - we can include the documentation online, expose it locally for use in a browser, and use it with context from within our application directly.