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+# Context and APIs
+
+GitBooks provides different APIs and contexts to plugins. These APIs can vary according to the GitBook version being used, your plugin should specify the `engines.gitbook` field in `package.json` accordingly.
+
+#### Book instance
+
+The `Book` class is the central point of GitBook, it centralize all access read methods. This class is defined in [book.js](https://github.com/GitbookIO/gitbook/blob/master/lib/book.js).
+
+```js
+// Read configuration from book.json
+var value = book.config.get('title', 'Default Value');
+
+// Resolve a filename to an absolute path
+var filepath = book.resolve('README.md');
+```
+
+#### Output instance
+
+The `Output` class represent the output/write process.
+
+```js
+// Return root folder for the output
+var root = output.root();
+
+// Resolve a file in the output folder
+var filepath = output.resolve('myimage.png');
+
+// Convert a filename to an URL (returns a path to an html file)
+var fileurl = output.toURL('mychapter/README.md');
+
+// Write a file in the output folder
+output.write('hello.txt', 'Hello World')
+    .then(function() { ... });
+
+// Copy a file to the output folder
+output.copyFile('./myfile.jpg', 'cover.jpg')
+    .then(function() { ... });
+
+// Verify that a file exists
+output.hasFile('hello.txt')
+    .then(function(exists) { ... });
+```
+
+#### Page instance
+
+A page instance represent the current parsed page.
+
+```js
+// Title of the page (from SUMMARY)
+page.title
+
+// Content of the page (Markdown/Asciidoc/HTML according to the stage)
+page.content
+
+// Relative path in the book
+page.path
+
+// Absolute path to the file
+page.rawPath
+
+// Type of parser used for this file
+page.type ('markdown' or 'asciidoc')
+```
+
+#### Context for Blocks and Filters
+
+Blocks and filters have access to the same context, this context is bind to the template engine execution:
+
+```js
+{
+    // Current templating syntax
+    "ctx": {
+        // For example, after a {% set message = "hello" %}
+        "message": "hello"
+    },
+
+    // Book instance
+    "book" <Book>,
+
+    // Output instance
+    "output": <Output>
+}
+```
+
+For example a filter or block function can access the current book using: `this.book`.
+
+#### Context for Hooks
+
+Hooks only have access to the `<Book>` instance using `this.book`.