I bought this book exactly one year ago, and I have used it for all that time. That's more than enough to make an opinion, and I consider Linux Kernel Architecture (LKA) a *very* valuable book, actually one of the most useful books about Linux Kernel that I own (and I think I have them all). I use it this way: When looking for some aspect of the Linux kernel logic, I first go to LKA to see if the topic is addressed somewhere (the answer is most often "yes"). Then I read the relevant parts of LKA before I start to dive into the source code for more. What I get is a plain English, detailed, structured functional and technical explanation of the code I'm interested in, with diagrams and figures whenever it can help. The book mentions the path of the relevant source files, something that saves time, too. In other words, I use it as a reference book, as a birds eye view into the kernel, but also as a functional explanation for a number of part of the code that are all but simple and obvious.
Now be warned: This is definitely not an introductory book for the beginner, nor a Linux kernel programming tutorial or techref manual, nor a book about device driver programming - even if it may help there. But all of those topics are addressed by other famous books such as Love's Linux Kernel Development 3rd Ed., Linux Device Drivers 3rd Ed, Essential Linux Device Drivers and a few others (I use all of those, too). This is probably not either a book that you would read cover to cover. It might not either always cover absolutely everything with all the details you might want (heck, it's only 1337 pages!)
But what LKA provides is a very well commented guide and roadmap into many aspects of the kernel, and one that is still recent enough to still be relevant. I just hope it will be regularly updated, and new editions republished. In other words, LKA is a great time saver. I consider my time as valuable, and LKA has paid for itself manyfolds.