Not OP, but the latter. The author talks both about data models (relational, document, graph-based, with some nice historical tidbits about IMS and CODASYL) and storage models (from B-trees to SSTables, passing through bitmap encoding and everything else).

It is an amazing book so far, and I'm pretty excited for the rest of it to be written. It's the best text I've read so far on databases that gives all the references you need if you want to go deeper, but still allows you to get a very good higher-level understanding without it.

It covers modern NoSQL databases but puts them in their proper historical context by comparing them to the (largely rejected) hierarchical database systems. (At least when we're talking about things like 'document stores.')