But honestly the official study guide still form the bases for your study and very good reference for you to start your study or exam prep.
I would suggest you look into the domain changes. Highlight the new /change one and try to see if you already have knowledge on those. I am sure the new ones are very common which you can find related knowledge easily.
9th edition is out now on amazon. As well as the latest version of the practice tests.
Im only starting the CISSP study myself too, so im no expert but I did see the video from the books author (Mike Chappel) who said that 5 our of the 8 study domains have had changes made to them. Personally, I also have the 8th edition of the book at home from last year but i bought the new one (edition 9) 2 weeks ago and will be working from that one.