Funny you should mention that. The third or maybe fourth was reviewed in the weekend paper (yes I went analogue) and sounded quite intriguing
here's the review, it is the fourth instalment
https://www.theguardian.com/books/2018/sep/14/labyrinth-spirits-carlos-ruiz-zafon-cemetery-forgotten-books-quartet-final-review
I remember the first instalment was hyped so much over here that I ended up ignoring it! I might try it now.
Do you read these novels in the Spanish or translation?
(Also, as an aside I passed on the Sid Lowe book about Barca/Real to a football mad friend today so your recommendation still bears fruit)