The Macintosh operating system was long considered more secure than Windows.  Part of the reason was that the vast majority of attackers targeted Windows given it was so ubiquitous. 

A lot has changed and the Macintosh operating system, currently known as OS X is both a target and highly vulnerable.

In The Hacker's Guide to OS X: Exploiting OS X from the Root Up, authors Robert Bathurst, Russ Rogers and Alijohn Ghassemlouei detail the fundamentals of the OS X environment and its many vulnerabilities.

The book is a like a slimmed-down version of Hacking Exposed for the Mac.  In that it both shows how to attack the operating system, in addition to detailing defensive measures to stop attackers.

While the Mac O/S has evolved significantly over the years, it is debatable if Mac O/S has evolved equally. 

OS X is debatably just as vulnerable as Windows yet it does not get the attention that Windows does.  That is changing quickly as the book details. 

For anyone having to secure OS X or iOS, The Hacker's Guide to OS X: Exploiting OS X from the Root Up is an invaluable reference.