I don't think IQ matters as much as effort. Those who read and reread the books and spend time thinking about the concepts will do much better than those who just skim (or don't read at all).

A smart person will get more out of the books on a first read, but I think a person with 100 IQ can squeeze just as much out of them over the long run. A very smart person who grasps the theory quickly and easily but who never really bothers to master its implementation can remain a big loser, while a relatively dumb person who works hard to internalize the principles and apply them to his game can be a big winner.

I've never read HOH, so this is based more my impressions of TOP, GSHE, and SSHE - but it seems like it would be applicable to most good poker books. I also find resourcefulness to be a better measure of success in general than intellect.