Apexトリガー上でYQLのデータを取得する ref: http://qiita.com/arufian/items/99a5e974549422ad0235

file0.java

global class YQLBridge {

    @future (callout=true)
    public static void searchGoogleBooks(String bookId) {
        Book__c book = [SELECT Name, Id FROM Book__c WHERE Id=:bookId];
        Http http = new Http();
        HttpRequest req = new HttpRequest();
        req.setEndpoint('https://query.yahooapis.com/v1/public/yql?q=SELECT%20*%20FROM%20google.books%20WHERE%20q%3D%22'
            +book.Name+
            '%22%20AND%20maxResults%3D1&diagnostics=true&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys&format=json');
        req.setMethod('GET');
        HttpResponse res = http.send(req);

        Map<String, Object> root = (Map<String, Object>)JSON.deserializeUntyped(res.getBody());
        Map<String, Object> query = (Map<String, Object>)root.get('query');
        Map<String, Object> results = (Map<String, Object>)query.get('results');
        Map<String, Object> jsonItem = (Map<String, Object>)results.get('json');
        Map<String, Object> items = (Map<String, Object>)jsonItem.get('items');
        Map<String, Object> volumeInfo = (Map<String, Object>)items.get('volumeInfo');
        String richStr = 'title :'+volumeInfo.get('title')+'<br >'+
            'subtitle :'+volumeInfo.get('subtitle')+'<br >'+
            'author :'+volumeInfo.get('authors')+'<br >'+
            'thumbnail : <img src="'+((Map<String, Object>)volumeInfo.get('imageLinks')).get('thumbnail')+'" />';
        book.Web_Results__c = richStr;
        update book;
    }
}

file1.java

public static void populateInternetBooks(Book__c book) {
    YQLBridge.searchGoogleBooks(book.Id);
}

file2.java

trigger PopulateWebResults on Book__c (before insert, before update) {
    Book__c[] books = Trigger.new;
    for (Book__c b :books){
        if(b.Web_Results__c == NULL || b.Web_Results__c.trim().equals('')) {
            BookController.populateInternetBooks(b);
        }
    }
}