A more important factor than genre?

While I think that Confessions being a personal history and Invisible Man being a novel makes some difference in how they present themselves and my reaction to/enjoyment of each, I believe the wildly different time periods they were written in makes a much bigger difference. I find that, as a general rule, the older a book is the harder it is to read. There's definitely exceptions to this, and I'm not saying that older books are not often very informative and full of wisdom, but in terms of ease of reading (and often although not always enjoyment of the book) the age definitely makes a difference.

This is often compounded, as in the case of Confessions, by the fact that many books that are more than a few centuries old weren't weren't originally written in English. I feel like no matter how good a translator is some of the author's intention, and quite a bit of the writing style, is always lost if one can't read a book in its original form. So honestly I don't think it would make that much of a difference if Confessions was written in more of a novel form, at least not to college freshmen who don't know that much of the background behind it. As for Invisible Man, it would be an entirely different book, albeit with some similar themes, if Ralph Ellison had set out to write his autobiography.