I highly recommend this book if you are interested in learning about computer science. The book is organized around the idea of building a computer from the fundamental logic gates up--starting with the hardware (combinational logic gates, arithmetic logic units, sequential logic gates, the CPU and memory) and then through the software hierarchy (starting with the machine language, and working up through the assembler, a virtual machine, a compiler for a high-level language, and an operating system). As a "by-product," one learns, by very relevant examples, many fundamental concepts of computer science.
You can just read the book, but the best idea is to follow the authors' advice and do the projects where you implement every necessary piece of the computer system yourself. The projects are all very well organized. All the software necessary to emulate any part of the computer is available for free download from the authors' web-site. It all works beautifully. If you want to skip any of the projects, you can, because the software is organized in such a way that it will use built-in modules instead of the ones you built if necessary.
The authors seem to have extensively tested the whole approach through the courses they have taught using this material. I also noticed that Harvard's Computer Science 101 course is being taught based on this book. I have been using the book for self-study with absolutely no problems--in fact I have never had such a great experience with a self-study course. All you need is a Windows or Linux (edit: Mac OS X works fine too) computer and access to the internet, and you can give yourself a wonderful education in computer science.
In terms of prerequisites, you only really need to have some experience with programming (e.g. with C, or ideally with Java or Python). I think that the book should work well for students or hobbyists who don't have any more experience than that, but it is also great for much more experienced students, as a kind of integrative summary of the field. I also think the book is perfect for graduate students or researchers from other fields who want to learn how digital hardware and software systems are actually engineered.
Finally, I just want to compliment the authors on the extraordinary care that they have taken with the whole project. The computer design that you build up is wonderfully elegant--at every stage the design is just as simple as it can be while being sufficient. Every piece of emulation software works as advertised. Even the extra powerpoint or .pdf tutorials are nicely done. This is really quality work, and using it is just a real pleasure. Finally, the source code for all the software provided by the authors is available, so if you wanted to extend the provided emulators, you could do that.
In summary, I give this book my unqualified highest recommendation.