Where to start?
The writing: lyrical and vivid don't do Zafon's skill justice. They are just words. My words. Sort of like someone giving you a brush and the exact colors that Degas used to paint a masterpiece. A nice start, but you won't get there. I will just say that very, very few writers have ever painted a picture like Zafon does. Old school, classical, timeless, beautiful. I feel lucky to have his books to read.
The characters: Dickens and Dumas would love these characters. They are complex, endearing, vivid...alive. Brimming with humor and menace, they leap off the page. It just doesn't get any better.
World building: you and feel the damp and smell the aroma of Barcelona. You can't get closer without a time machine.
Tone and style: Gothic with a sense of humor and rapier wit. You will run out of note cards on which to copy quotes. I really feel that Mr. Zafon is a modern Dumas. What is it about these Spanish writers (also thinking of Arturo Perez Reverte) that makes them so good? They are throw-backs in such a good way. Every time I read them I feel that someone discovered a great lost Dumas novel in their attic.
Story: seamlessly weaving into the prior books, it made me want to re-read them just to experience them again in light of this wonderful new story that provides both a present story and a back story to a central character---so the action both follows AND predates the other books. Don't be afraid to read them out of order, but you really should start with Shadow of the Wind as it really sets the whole Zafon world in motion. My only gripe? I was done too fast. You fly through Zafon's books and while it was a satisfying story, I wanted more. He is the only writer that I can think of that I wish every book that he wrote was 1,000 pages long.
He may not be to everyone's taste, but if you like gothic, atmospheric, old style dark adventures; if you really love Dumas; if you love to visit vast, ancient cities and meet characters both sinister and angelic, then you will love Zafon's books.