I took the exam nearly 20 years ago, but a year ago I was working with someone who was prepping to take it, and it struck me that a lot of the content hadn't changed that much. I think the big changes in the past two years or so have been in the exam format itself - the introduction of the adaptive test and then I believe they've also added some experimental questions.
I wouldn't fret too much about the difference in the editions. There might be some updated content from one to the other, but the majority of the content (at least from what I observed) is pretty timeless. Put another way, if you don't pass the exam, I don't think it would be a matter of having used old material. Heck, I'm old material, and I am still managing in this profession 😉
The challenge of the CISSP really falls into the breadth of the content, in my view. In that regard, I'd really come at the exam from multiple sources. Maybe two different test prep books, but also add some reading/viewing on topics/domains, especially the ones you're not strong in. In the end, you have to return to the official study guide. As you will learn, the ISC)2 might choose wording that's different from other sources. And again, that's something I don't think they've really changed over the years.
