Automatically tag posts from Kindle book imports, see: http://www.reddit.com/r/Wordpress/comments/salk3/plugin_to_create_tags_based_on_specific_text_area/

kindle-book-auto-tags.php

<?php
/*
Plugin Name: Kindle Book auto-tags
License: GPLv2 or later
*/

function kindle_book_auto_tagger( $post_id, $post ) {
	// Check for the string with our tags
	// Example string: "Customer tags: suspense, kindle free book, science fiction, post-apocalypse, fantasy."
	$regex = '/customer tags: ?(.+)\./i';
	preg_match( $regex, $post->post_content, $kindle_tags );

	// Make sure we found tags to use
	if( ! $kindle_tags )
		return;

	// Set the tags
	wp_set_post_tags( $post_id, $kindle_tags[1], true );
}
add_action( 'publish_post', 'kindle_book_auto_tagger', 10, 2 );