Eli Bendersky's website Yay, my first serious book in Spanish. Not the first overall, because I read a smallish (25 pages) book for beginners, but certainly the first /real/ one.
I admit, I've never read Harry Potter before. I know some consider this as falling behind, but my excuse is that I waited to read it in Spanish. And, now having read it, I think I made a good choice. Harry Potter is a great book to learn Spanish from - it's easy going, interesting, and the language is not too high. I came to a point where I understand what I read without a dictionary, except for a few sporadic words per page that don't ruin the comprehension. I feel I'm now much stronger in Spanish than before I started reading it.
As for the book itself - it's certainly entertaining. I wouldn't rate it anywhere near my "best books" list, and those who have the nerve to even mention a comparison to LOTR are delirious, but this book has its merits, and its popularity can be understood. The writing of JK Rowling is fast paced and flowing, funny and easy to follow. The level of the language is higher than I expected, which is good because the book is so popular with children - it lets them improve their vocabulary a little.
I think it's needless to mention that I found it to be quite childish, though given the audience, that can be understood.
Anyway, I plan to buy the 2nd Harry Potter book as well - to keep practicing my Spanish. I wouldn't waste my reading time on Harry Potter if I wouldn't be able to find it in Spanish. As a book for language learning, it's great (my friend perfects his French by reading and rereading its translation).