It depends on what you mean by designing operating systems. Usually I've found that OS taught in college tend to be out of date, that is of course unless you are grad/post-grad CS research.

Yeah its quite out dated but I think just a textual reading doesn't give us exact idea about the OS and its functions.

Try this book ( http://www.amazon.com/Operating-Systems-Design-Implementatio... ).

It is written be Andrew S Tanenbaum, the author of minix. The book contains whole of minix source code. Nothing beats learning from reading actual codes. Linus Torvalds was inspired by this book. In his autobiography "Just For Fun", Torvalds describes it as "the book that launched me to new heights".

Thank you I will surely go through the book.