I like Microsoft Word 2010 Bible by Herb Tyson. Even though it is for Word 2010 I think it is better written and more authoritative than the 2013 version. The differences between Word 2010 and 2013 for the general writer are minimal.

Styles are a key to formatting using Word. Here are two online references:
Understanding Styles in Microsoft Word
Tips for Understanding Styles in Word by Shauna Kelly

For your medical reports you may be interested in AutoText and other Building Blocks. Automated Boilerplate Using Microsoft Word

A good starting place is Basic Concepts of Microsoft Word - from Shauna Kelly.

Also, there are good video tutorials available through Lynda.com.