Which is the best and most used PHP Framework and where could we get its resources (Ebooks, training material etc) ?
"Best" depends on what you want to do really - it will differ depending on your needs. You can see a quick comparison of them here; http://www.phpframeworks.com/ Zend and cakePHP are however the most popular.
Personally, I would go for Zend only if you are using Magento, or you plan on trying to get a php job with a company. While it is made by the company that maintains php, it's overly-large and is more a group of libraries than a traditional MVC framework. The documentation is also horrible, which is where symfony and codeignitor blow it away. For personal usage, either codeignitor, or symfony. Symfony is #2 in php frameworks to Zend. Codeignitor and Cake, and Yii are the other top 3 last I knew. There's no best framework. Which to use should be dependent on your needs and your ability to code. If you just want pure speed, codeignitor is probably unbeatable. For features go with Symfony or Zend. If you plan to get a php job in the future, definitely Zend. Personally not a huge fan of Cake, as it doesn't offer anything unique or superior to codeignitor or symfony. In any case, there is a steep learning curve to learning enough to really get proficient at one.