Consider all of the books a way to help you understand what could be covered on the exam including terminology. In theory, no study materials should be needed at all if you are a true season professional. However, given the amount of terms and acronyms alone, there is little chance even a pro could pass the exam without some amount of studying. Likewise, if you only have book experience, you are less likely to fail the exam. Both of these are just my opinion but consider the questions you will be asked may not be simply terms and definition questions and require you to answer with best/most/least type of estimates. Knowing the terms is important too but may not be enough for most of the types of questions you could get on the exam.
IMO, the official books are needed resources to reference. They may be hard to read and may not provide enough examples or expand as much so other sources are often suggested.
In many study groups including this one, you will notice that there are variations on what each person that took the exam claims to have used and may be slightly to greatly different from one person to the next. Some people hate reading (...raises hand..) and some people get the most out of reading. For most, having more than one study source is recommended including books, online courses, audio sources, flash cards, practice tests, study groups, etc.
My suggestion to you is to start with what you know you will read from cover to cover and then supplement with other testing sources including practice tests. If you need more books to help you then go from there.
The official CBK and study guide are the two books I would make sure you have in your toolkit no matter what. The practice tests are the same as what is on the official CISSP study app. So if you are planning on getting the app, consider the book only if you like that format. I have both and the book is nice to get away from screens and more real estate for some of the bigger questions with diagrams.
Good luck!
