I recommend Jonathan Strange & Mr. Norell. 
 My biggest disappointment is the Raven King. All those talks about John Uskglass made him the character to anticipate in the last part. And then... nothing. What is frustration. But Susanna Clarke has other plans (on how Strange & Norell worked their differences), so the ending still went well for my taste. 
 Am I a Norrellite or Strangite? Neither. 
 Norrell is flaky for his age, his servant Childermass is more interesting than him. Strange is too ambitious for his own sake. It's sad that Strange lost the most important thing without him even knowing it, but when he did, it was too late. 
 It's funny, that when JS&MN started to bore me, those short (and often long) footnotes snags my attention. The side stories there are riveting and very, very imaginative. These parts are enough to make me go back and resume my reading. 
 Parts II and III are fun, especially when Strange starts to take interest on the faery roads (my favourite part). Everything faery here is fascinating, the tales so unreal and a bit scary for those beings we considered all cute and dusty with faery sprinkles. The Kingdom of Lost-Hope is enough to make me cringe with nervousness. 
 I would not have read Jonathan Strange and Mr. Norell if not for the buddy read that I joined. It's thick, and thicker every time I put it down (it's so heavy), because I honestly thought I won't be able to finish it! 
 Patience, lots of it. It is the key to enjoying and finishing this brick fantasy. But I believe when you get to the part where Strange is using his learned magic to win a war (and later, to win back his life), you will get hooked. That's what happened to me. 
 Last piece of advice, then: Good Luck! *grins*