Shon Harris, RIP, material is still relevant! I found I understood her writing style. Helped explain a lot where I needed a deeper dive. You should know how a block cipher differs from a stream cipher. You won't need to get technical such as CLI configuration commands on a Cisco switch, the details with exploit tools with Metasploit, etc. CISSP is not CEH, more like CASP.
You should know things like TCP flags, what they are, and what the 3 way handshake is, and also what kind of vulnerability's it has. Mnemonics like Unskilled Attackers Pester Real Security Folk help out, and basically is the extent to know for the exam. Know about sub-netting, mask, the way it's identified with a /24, but you shouldn't worry about solving binary math network/host questions.
The CBT/CAT? is the way to go. Cut down from a 6 hour to a 3 hour time limit. You'll have to answer somewhere in the neighborhood of 100 questions.
Take a look at Sari Greene's video for a good explanation of reading and understanding CISSP exam questions.