My book "Unix for Mac OS X" is now out in a new edition, completely revised for OS X 10.4 (Tiger).
This book is a very complete beginner's guide to Unix. The book assumes no prior Unix experience and yet provides more detail than any other beginning Unix book of which I am aware. In my not-so-humble-opinion the book is probably the best beginners guide to Unix on the market.
The book is over 500 pages and provide great detail on the Unix layer of Mac OS X (aka Darwin.)
The book teaches you how to get to the command line, to edit files using a command line editor, to deal with Unix permissions (an entire chapter on that), and so on. I'll post a table of contents in a later posting.