First, in the interest of full disclosure, I will tell you this: I have not read the Harry Potter books. I just haven't. I read about half of the first one and just...didn't....finish. Sorry. I'm sure they're very good books. I'm glad you like them and I really should read them. It's not you, it's me. But I have an excellent reason and that reason is John Bellairs.
Long before your boy Harry was but a twinkle in J.K. Rowling's (filthy, filthy rich) eye, Bellairs's boy heroes like Lewis Barnavelt and Johnny Dixon were sneaking through Gothic mansions and dusty libraries and secret tombs, their flashlights, coded maps, and ancient spell books in hand. And, lo, it was wonderful. (If you aren't familiar with his work, John Bellairs wrote a series of young adult Gothic mysteries, published between 1973 and the early 90's, that were usually set in mid-century New England. More about him over here.)
I think I started reading these books in middle school after discovering them in the youth section of my small town library and being instantly drawn to the Edward Gorey covers and illustrations. (Edward Gorey Side Note: Occasionally, when allowed to stay up a bit late, I would catch the animated Gorey opening to Mystery! on PBS and think excellent! this should be really, really spooky only to discover the ensuing program was just British people talk talk talking and driving around in the countryside a lot and where were the warlocks? and how about some ghosts, at least? and where the heck were the mysteries, exactly? and I'm going to bed.)
And even better than the illustrations were the stories. The stories had all the right things: creepy atmosphere, obscure historical references, truly evil villains, dangerous expeditions carried out by normal kids, grown-ups that just wouldn't believe said kids knew that the fate of the mortal world was hanging in the balance, etc. This was addictive stuff. I used to consume each book in a few days, pushing through chapter after chapter, staying up late into the night on the sly, leaning down to the foot of my bed, catching the pool of amber light from the hallway just so, quietly slipping the book under my quilt if a parent was coming, lying in the dark with the crap slightly scared out of me by whatever I'd just read.
And this was not sissy stuff. The stories were genuinely scary. For example: go read the plot synopsis for The Curse of the Blue Figurine. I'll wait right here.
See? That's scary, right? (Priest joke omitted.)
For me, growing up in a small, sun-drenched, Idaho agricultural town, these books were pure escapism. There was a painful lack of Gothic American castles in my neck of the woods. And no woods, for that matter. I so dearly wanted to find a mysterious talisman in a church pew. A yellowed map, inscribed with hieroglyphics and Masonic symbols, maybe? How's about a cursed signet ring? Even the public library that introduced me to these books in the first place was an enormous disappointment. The libraries in Bellairs's books always had an Occult section tucked away in some dimly-lit basement, while my library had a really extensive, well-lit Lewis & Clark section and an embarrassing amount of romance paperbacks.
And I think, because these books so powerfully captured my imagination as an impressionable young thing, there just wasn't enough room in my heart so many years later when that Harry Potter boy came along. Sorry, J.K. I belong to another, and I don't think you need the cash.
So, in summation, read John Bellairs's books. Start with The House with a Clock in its Walls. If you have or know older kids, check out or buy these books and read them together. But don't buy the new editions because the cover illustrations suck. And read the books at night. Maybe on the sly. Especially when it's stormy.
And if you do read them, or have read them, tell me about it, and our nerdy Bellairs love will shine for the whole Internet to see.