What makes a book hard for you? Lots of people thought Jellicoe Road was hard, with its two plots that you're pretty sure tie together but you're not sure. People struggled with Bog Child, I think because of occasional dialect/slang (although no where near as crazy as Trainspotting. I had to read that sucker out loud for the first half so I could understand the dialect) and the history. People thought Octavian Nothing was difficult due to the slowness of the plot and of course, the language.
I didn't find any of them overly difficult. They were all harder than Twilight of course, and even more difficult than The Book Thief or Goose Girl, but I wouldn't have described any of them as hard.
Right now, I am struggling through Dragons of Babel. I find this book extremely difficult and I bet most readers don't. I'm struggling with it for several reasons. One big one is I don't read a lot of non-urban fantasy. I am unfamiliar with the conventions of the genre. There are several things I'm pretty sure I'm missing because of this. Despite this being "high fantasy," elements of my known world sneak in--cigarettes and cars and even music (Mozart and Ellington amongst others) which throws me.
The big one is this--Swanwick really doesn't explain the backstory (or at least he hasn't yet and I don't think he's going to.) What this war is about and the politcs of Avalon and I'm pretty sure that Babel is the capital of Avalon, but it might be the capital of the invading country that's swallowing Avalon whole. I have no idea. I don't know what half these creatures are (but I think I would if I read more fantasy) and so much is unexplained. It doesn't really need to be for the plot, but most authors would explain it anyway. Most authors would take their readers by the hand and fill them in on what's going on in the broader picture in pages or just a few sentences, but to give them a much more solid grounding in this world they built. (And don't get me wrong, I'm not faulting Swanwick's world-building at all.) At the least, they would give their readers a map. Swanwick doesn't do this, not because he's being coy or messing with his readers, but because he trusts them. It's like he's saying "you guys are smart, you can figure out enough without the filler" and he's right. I love him for it.
I might understand more if I read more fantasy, but I love the trust Swanwick puts in his readers to figure out what they need to know.
That said, this probably wasn't the best title for me to pick up from this year's Alex Winners when I'm trying to read as many books as quickly as possible. I only picked it up because it was the first Alex to come in for me at the library when I put them on hold. I'm really, really glad I did.