I think it mostly boils down to how interested you are in the backstory of the Warcraft universe. To me the backstory is what I enjoy the most so I tend to like the books. I consider World of Warcraft to be mostly a theme park where I can go visit the locations from the books (although the world map is severely abridged for gameplay reasons)
From the ones I've read I can recommend Lord of the Clans, Day of the Dragon, Of Blood and Honor and Rise of the Horde.
Cycle of Hatred had some good scenes with Thrall and Aegwynn but all in all the story didn't really amount to much. The Last Guardian was decent but it had some dodgy writing.
I also started reading the War of the Ancients Trilogy but I just couldn't get into them.