Learning to Program to the Active Directory is made easier by choosing the book that is right for you, you could start on the 'Net by immersing yourself in msdn.microsoft.com and the AD SDK material but if you are like many people you need to 'have the right info at your fingertips' and this MS Press book by Charles Oppermann contains the right dose of material to assist you in your AD-enabled project! Other books such as Kirkpatrick or Robinson dive into different aspects of AD programming in more depth (especially if you are C++ inclined) while Oppermann chooses to cater towards both beginning and experienced AD programmers.Many programmers will discover some new pointers and 'best practices' that they will find to be of use!.
If I could cherry pick among the various AD/ADSI books available I would augment Oppermanns excellent book with Chapter 18 of the O'Reilly book by Alistair G.Lowe-Norris on Permissions and Auditing.
In short even if you are now moving to System.DirectoryServices under .NET you will want a copy of Charles Oppermanns' Active Directory Programming for your AD programmers toolkit!