This book was written about the time that Ritchie and Thompson began work in UNIX. I was curious what ideas were floating at the time that didn't make it into UNIX. It turns out that most of the topics in this book did eventually end up in UNIX.
There is a long, and depressingly up-to-date, discussion of database implementations from Jim Gray (and the statement that database implementers would prefer to use general operating system facilities for what they do, but sadly the operating system facilities generally don't fit the problem well; that statement is also depressingly accurate today).