Unfortunately, due to sandboxing rules added around beta4, WKWebView doesn't allow access to any local files, making it nearly worthless for hybrid apps, ebook viewers, etc.
The only workaround that I know of is to run a webserver on a background thread and point the WKWebView at it. Hopefully Apple fixes this in a future release.
Also, WKWebView doesn't work with NSURLProtocol, which a lot of companies use to implement encryption and dynamic (e.g. database-sourced) web pages.