For resource of WordPress, WordPress Codex is available as WordPress official online manual.
WordPress users all over the world post and update an article every day. There is a huge amount of know-how about WordPress.
A lot of blogs and books are using the site as a reference. This site is very useful for you.
WordPress Codex: http://codex.wordpress.org/
WordPress Codex Japanese Version: http://wpdocs.sourceforge.jp/
WordPress is an open source program. There is no document provided by the developer, but WordPress users themselves are making the document together. The document includes WordPress source code, function under development, and so on.
And since the WordPress Codex is written in English, the volunteers are translating it, and making the another language version.