I started studying for the CISSP seriously around the end of summer 2017. I've been on the fence about ever taking the CISSP exam for quite some time due to the sheer amount and broad range of subject information. I finally decided to make an attempt due to the upcoming changes just a couple of weeks ago.
ISC2 Official CISSP CBK (older version when there were still 10 domains)
Shon Harris' All in One guide & practice test questions
I didn't particularly care for the way ISC2 presents their information (personal preference, their materials are quite thorough). Like most candidates I took numerous practice tests. As some others have mentioned the actual test questions were different than what I experienced when taking practice tests. I feel that this is a good thing as it makes test takers use their critical thinking skills and better assesses their knowledge. This also better prepares us for real world situations where there is no text book answer. I believe the purpose of practice tests is to help you identify the areas that you're weak in so you can devote more time to studying that information. I also feel that there is no such thing as being overly prepared, it's always great to have a mindset of continuous growth.