This book is a wonderful example of how to write a clear, concise, informative text book. The style is easy to read, covers the appropriate level of detail at every stage and manages to pack in a fair amount of information. Best of all, you get a compact, well documented UNIX clone suitable for using, expanding and learning from.
I spent a lot of time at university hacking the provided source, tweaking, changing, testing, expanding and there's no better way to learn about operating systems than to have the MINIX source and this book side-by-side.