See if you can find topics that are not covered in the language documentation. See if the book seems to give selected topics more attention than the language documentation. If there is a beginner's chapter that corresponds to the one you read in the language documentation, read it through and see if the author offers any insight or information that you did not get from the language documentation. If the book does not meet any of these criteria, you may be dealing with a manual rehash. Frankly, I don't think I've ever written a chapter in seven hours even when I knew the material.
Most chapters require additional research and sample code.
There are actually some good reasons. The author may be an excellent programmer and writer who is looking for an opportunity to establish credibility as a writer. The author may be interested in the publicity involved. The author may be using the chapter to promote a company or product. Unfortunately there are some bad reasons as well.