If one is already well-versed in multiple areas of software technology (especially development and database administration), this is an excellent book. It surveys the landscape of software data storage technologies, talks about (at a modest level of depth) some of theory behind things like quorums in distributed database systems, resiliency/redundancy strategies during data loss, and a host of other interesting topics.
I'd consider its level of depth somewhere in the middle between specialist books and 10k foot overview books. I recommend it to anyone that has been a software developer or DBA for 5+ years, as I think they'd get the most value out of it.
As an engineer new to system design, I found the whole book to be gold. It gave me the vocabulary to continue learning more on my own.
[1]: https://dataintensive.net