Also see
this one, Gary. Both are available together as well on the link you posted.
I buy from Tony Hill frequently, especially the exotica Penzey's et al. don't offer. I imagine the book would be worthwhile, as would Norman's.
I know of no material specific to spice/herb combinations. Though I agree with Purviance that combinations are subjectively appreciated, I think there are guidelines (many general but more than a few that could be considered specific) that one can employ. Much can be gleaned from trial and error, as Paul suggests.
I think that 'balance' has its place and I often shoot for that, but I also think it can be overrated--it depends on what one is shooting for. For example, a nicely balanced rub on ribs might get totally lost when the ribs are sauced--even if the sauce was balanced. It might be necessary to skew the rub one direction ('unbalance' it, if you will), the sauce in another, so that when combined balance is achieved. And, of course, items like lemon-garlic rice, shrimp fra diavolo and honeyed babybacks are skewed, respectively, toward lemon-garlic, heat from crushed red pepper and honey. Still, one doesn't want to go over-the-top so an eye toward balance while leaning a particular direction is warranted. As Paul aptly notes, sometimes fixing a problem means the focus needs to be on balance.
Some stuff you might find helpful: See my posts on
this thread;
this one on quality and chilies; and
this one on the use of smell.
Hope this helps.