Guess I missed that book. ;-)
There is no rule of 30 or 50 or 60.
Google (and I think Y too?) have implied at times that there should not be more than 100 outbound links, and upon being queried, clarified to suggest that page size should not be more than 100K.
The simple facts are these: The juice (commonly thought of as PageRank because of G) that is passed on to other pages is essentially divided among the number of outbound links. More links means less juice being passed on to each target page from the source page. So to the extent that one is concerned about passing more versus less juice to linked pages, then fewer links is better.
But passing PR is only one of numerous factors that should determine how many links go on a given page.
Now that pages can be constructed with far less code than was the case five years ago, and therefore are not nearly so heavy, a resource page that is well organized can easily be highly usable for the user (and beneficial to linked sites) with 80-100 outbound links. In fact we're in some industries where some of the most sought after links come from sites that are organized like that.