This is actually a pretty difficult question to answer.
Depending upon your current level of experience and 'how' you want to approach the platform, different texts are going to be more relevant to you than others. Similarly, different people have different needs/wants from the Mac so naturally, the frame of reference that people approach a new topic/subject is going to vary. Software developers will of course see the platform differently than musicians; graphics designers will see it differently than engineers.
David Pogue's
Missing Manual series is quite popular on a general level. I would probably recommend starting here first.
The other recommendation I would make is checking out the
Peachpit Press Apple Training Series. While these are primarily support type manuals geared towards people seeking Apple certifications, they can be invaluable in teaching the platform in general since they are geared towards professionals wanting to support the platform.
If your interested in the nuts and bolts, low (and I mean very low) level details and you have a fair amount of software/hardware development in your background you may want to look at Amit Singh's book
Mac OSX Internals: A Systems Approach. However, be forwarned, this book is not a beginning or even intermediate text. This is an advanced work and is largely targeted towards computer science students or experienced software developers. Having said that, the first chapter on the history of Apple and it's software is very entertaining. Rather than buy the book, you can download a PDF copy of the first chapter.
Hope this helps.