Normally I would agree. But not when it is about computers and programming. I did a simple search for books containing the word PHP in the title and was published 2006, and got 25 results. About half of the books contain both PHP and other stuff like MySQL. But it still leaves about one book per month, making me think that there have to be a books that are new and cover PHP 5.2 that was released in november 2006.
Of course, if there are a really good book that is a year or even two years old. Of course the reader have to understand that there may be changes that are not covered. Heck, I am using a book from 1999 that covers HTML, but I know that I can only use it for the basics.