Unless I've missed something, a web browser "Find in Page" doesn't come up with any mention of the beta aspect on the hop pages on How to Brew. I'm curious myself as to the beta aspect, though I will just Google it now
*EDIT - well that was fast. I did a search and found a wiki on hops that does a good just describing the beta aspect. This part is the quickest most useful though the article talked a bit more on it:
"Beta acids do not isomerize during the boil of wort, and have a negligible effect on beer taste. Instead, they contribute to beer's bitter aroma, and high beta acid hop varieties are often added at the end of the wort boil for aroma. Beta acids may oxidize into compounds that can give beer off-flavors of rotten vegetables or cooked corn."