Having just packed, I can tell you books were great - not that they account for 80% of my stuff, but in similar Pareto-ish fashion they were very easy to slide into boxes and make a big visible impact on packing process.
It's all the fiddly little things that take time (do I even need that) or things that need to be well protected - glasses etc. And things that you frequently use and deliberate over whether you need again before the move.
Books are heavy as shit though. If I take two similarly-sized boxes and fill one of them with books, and the other one with random stuff, the box with the books is going to be the heavier one.
Pretty sure the densest box that I had in my last move was the one where I had all of my books.
It's all the fiddly little things that take time (do I even need that) or things that need to be well protected - glasses etc. And things that you frequently use and deliberate over whether you need again before the move.