I may be behind the times, but I never heard of hard fantasy before. In fact, I only discovered this sub-genre on a Goodreads discussion forum. (One of those where people try to figure out what the difference is between science fiction and fantasy.)
The person provided quite a few links, including a wiki article. Wikipedia says the Recluce Saga, a Song of Ice and Fire and Magic, Inc. are examples of hard fantasy. Looking through Goodreads and LibraryThing shelves, people have also tagged Lord of the Rings, the Farseer trilogy and The Family Trade as hard fantasy.
There is an article that was posted in 2008, but I’ve never heard of hard fantasy until now. Well, shows what I know, huh?
From what I understand, hard fantasy is the fantasy where magic has rules. Truthfully, I am stunned there is even a sub-genre for this.
Though, yeah, the magical rules of most fantasy don’t have scientific rigor. Some books do treat magic just like it was a science, have schools and everything. Though truthfully, of the books I’ve listed here, only the Recluce Saga comes close to doing that. The others? I don’t know.
I haven’t read a lot of hard science fiction. Maybe that’s where my own disconnect is coming from. But most science fiction don’t have a lot of a scientific rigor, either.
But since people have tagged them as hard fantasy, I don’t think I understand what makes a book hard fantasy at all.