Many people who are commenting on the book by Tanenbaum are people who are writing after the year 2000. Operating system developed has had a long history since 1970 when Unix was developed.
After that there was no significant movement on the Unix like systems (of course Microsoft and Apple were working) on their own OS but there were not Unix like.
Tanenbaum wrote a Unix like System in 1987 which shares the File management, Process Management, Device Management and Memory management with Unix. He called it as MINIX OS.
After that rest is History, Linus Torvalds wrote the Linux kernel. Even though Linux today has advanced a lot due to the Networking code, ACPI, SMB, Device drivers, please should examine Linux 0.01 written first by Linus.
That code also consisted of only 20,000 lines and was very similar to MINIX(Not in design but in Code size).
So do not blame the Author if he is not writing an advanced OS for you to study. Those have millions of lines of code.
What MINIX shows us is a reliable and stable OS that is good study material for better OS's.
If you want to understand nuts and bolts of OS study this book and write the code. You will be a master of Operating system.
Atleast it is not those books who dig into a LOT OF THEORY like Different types of OS and explaining the theoritical aspects but do nothing of implementation.
This book does the implementation. * Stars.