Mac OS X Unleashed devotes about 1000 of its 1500 pages to the BSD subsystem of OS X, and goes into great technical detail. It's preferable to a generic BSD book as it takes into account Apple's deviations from the BSD standard. Find it at Amazon
Note: The second edition of Mac OS X Unleashed, which covers the substantial changes made to the unix subsystem of 10.2 (Jaguar) was published in December 2002.
And the third edition covering 10.3 (Panther) was published in January 2004.