I’m not a reading specialist, but I can tell you this: Kids with ADD (who often struggle with reading, though not always) are motivated by novelty and interest.
If they LIKE the subject matter, they’re more likely to read. If they’re bored, they’re less likely to stay engaged.
Reading takes a lot of effort for some kids, so it’s gotta capture their attention enough for them to do the hard work of reading.
In some ways, then, the kind of book a child reads is not very important – what matters is whether its relevant to the child who is reading it. Comic books are every bit as helpful as chapter books — if it inspires a child to go to the effort of reading!
Now I’m not saying that you shouldn’t also encourage your child to read quality literature and stretch his vocabulary. But if your child is struggling with complex material (especially if he’s a young reader), consider reading aloud, or letting him listen to books on tape. Focus on helping him learn to enjoy the process of reading first, before you “make” him do the “hard stuff.”