+1 for DDIA.
You see you can create a basic database using 4 Linux commands: echo, grep, sed, and awk.
Then why are 1000s of databases in the market? And how are they different from each other? The book goes into each of the layers of the database: storage engine, query engine, encodings, indexes, transactions, replication, sharding, etc and discusses the advantages and limitations of various databases so that you are in a better position to understand whether your choice of database solution is the right one or not.
This is not a book to be read once and say you're done with it. I have read it 3 times and every time I found something interesting that I faced as a problem at some point of time. I still don't think I have been able to master all the concepts. ;)