I studied on Mike Chapple's Official Study Guide, 8E. It has now been replaced by the 9th edition.
The learning layout of 8E was completely satisfying, and the practice tests were thorough.
For the eighth edition, you could only use the online portal and take the practice tests for one year, once you register the book with Wiley. If that is the same with the 9th edition, plan accordingly.
Good luck to your study group!
Not exactly a CISSP guide, but I think Ross Anderson's Security Engineering is a worthy addition to any security bookshelf.
Agreed. A very good book. Not useful for certing, but illuminating in the realm of security.