Johnson is not a mathematician or a computer programmer–he’s a musician who dabbles in both just to find compositional ideas, so none of his algorithms are very difficult to suss out. The math is pretty easy, as are the computer algorithms. And Johnson is a very clear writer. That review you linked is actually more mathy and complex than most of what’s in the book.
Johnson likes finding simple little algorithms that produce interesting musical results. He feels that the more complex the algorithm, the less immediately musical the results. This means the musical ones are easier to understand.
You don’t really need any musical theory to understand the algorithms, because they’re all melodic. (He has another book on harmony.) It does help to be able to read music. Johnson assumes musical literacy, in order to show you on paper what the results sound like. But one could implement the algorithms and then find out what they sound like that way.
You might look at the articles he has on his website to get a flavor, although the book is generally better. Also try to get the book through Inter-Library Loan before dropping the $35 on it. I bought it new and have no regrets. I’m musically literate, but have none of the math on which the book draws, and only very basic programming chops. I was able to understand 95% of the book without much trouble.
