"Learning Flex 4" explains all of the core concepts for developing Flex 4 applications. It is definitely a beginners book as it assumes only some basic programming knowledge, and the book does nott really go much beyond the basics of writing a simple Flex application.
The book is well written and uses many examples to explain the core concepts of Flex application development. The book would be a good resource for someone completely new to Flex development and ActionScript, but it lacks depth for someone who already has a decent understanding of Flex development. The book also seemed to be rather wordy, and it took a while to get through even already understanding the majority of the content. Overall, I do not think this book is worth the money because it spends all of its time explaining the most basic level concepts of Flex development. Adobe has already put together great documentation and training materials on their website (see link below). I highly recommend these materials (which are free) over purchasing this book which contains pretty much the same content.