From the preface, it looks like a bunch of the reference material has been cut from the book in favor of C++ documentation on the internet:
"The third edition of Programming: Principles and Practice Using C++ is about half the size of the second edition. Students having to carry the book will appreciate the lighter weight. The reason for the reduced size is simply that more information about C++ and its standard library is available on the Web."
To be fair, someone should print the online reference material to get the real comparison.. it's not like we have fewer details in the standard library -- footnotes about compatibility with which version(s) and the related semantics could fill a book on its own.
"The third edition of Programming: Principles and Practice Using C++ is about half the size of the second edition. Students having to carry the book will appreciate the lighter weight. The reason for the reduced size is simply that more information about C++ and its standard library is available on the Web."