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Good question. There are two ways of looking at this. On the one hand, you're buying an electronic version of "The Yii Book" by Larry Ullman, in English. You will be able to download as many copies of the book as you want in these electronic formats: PDF, ePub, and mobi. All are DRM-free.
Moreover, when updates of the book are published, you'll be able to download those, too (at no additional cost). This includes all updates...
Another way of looking at this is that you're rewarding Larry for whatever benefit he's given you over the years. This could be: