In the case of this book, there is no word anywhere regarding which version is covered. It is clearly a 2.6 book, but that is all we know. Your editor has come to the conclusion from his reading that the book was a long time in the writing (not surprising: the subject matter is complex, and the book is over 1,000 pages long), and that, if an effort was made to make it consistently current for a specific kernel version, that effort was incomplete. The section on interrupts, for example, presents the old prototype for interrupt handlers last seen in the 2.5.68 kernel. Other parts are much more current. The book is a bit of a patchwork in that regard.