I'm interested in both. I have the Stronghold Builder's book, and I think it's a very useful book. I get the feeling the Book of Challenges will also be very useful. Perhaps the fact that both seem more utilitarian in nature than the average splatbook makes them less likely to be fodder for lively debate. At least with the Stronghold book, the stuff in it seems less open to misinterpretation than, for example, feats or prestige classes.