Like the second edition before it, the third edition of Advanced Programming in the UNIX® Environment has been updated to reflect contemporary operating systems and recent changes in standards. Obsolete material has been removed, except where that material provides a useful historical perspective. The four platforms used to test the examples in the book include FreeBSD 8.0, Linux 3.2.0, Mac OS X 10.6.8 and Solaris 10. These platforms are a moving target, and most likely there are newer versions available now, so your mileage may vary.
Major changes include the addition of POSIX asynchronous I/O, POSIX semaphores, spin locks, and barriers. In total, more than 70 new interfaces are covered. In addition, most of the STREAMS-related material was removed, because STREAMS interfaces are now considered to be obsolescent by the Single UNIX Specification. If you need information about STREAMS programming, check out UNIX System V Network Programming.
See here for a complete Table of Contents for the third edition.