One of the reasons why relational databases are quite clear to me is because I paid attention during the "DB fundamentals" university course many years ago and because the book we had was gold. I think I still have it somewhere in the attic at my parents home.
I just googled and the book apparently still exists and has been kept up to date: Fundamentals of Database Systems, 7th Edition
It's a university course book and goes pretty into depth of DB design, so don't be fooled by the title, it's not a "tutorial" book.
If you want something a little bit less "theoretical" I've heard good things from this: Relational Database Design Clearly Explained
I have said in another thread that I don't buy tech books that much lately but DB design is one of those topics that are not a fad or tied to a specific language and for which a book is definitely worth it. Even if you use a multi database application (or based on a non relational DB) it's worth to have strong fundamentals on how to structure and relate data.
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