I have not yet meet anyone who can honestly claim that he/she can master everything in MS Word; however us knowledge workers have been use it so often that we should know better (regardless you like Word or not, as long you use it).
Word processing is definitely a under-minded, but most-to-have basic skill. This book 《Word 排版藝術》(in traditional Chinese) is worth the money to put on the shelf. For experienced users, the book describes those more advanced skills you might already know. By having the book, those skills become a "sure thing". You become a true professional who know how to use the software in a professional way (To be honest, I am sure you will pick up some tips that you did not know before).
For those who is yet a powered user, this is a must read book. From secretary, proposal writer, engineer, or book writer, as long you need to word-processing, I am sure you will learn the more then needed skill and for sure it will pay back some other days.
In short, unless you will never use M$ Word, study on and get serious with what you're doing in document production.