If you really are only interested in currently canonical books (and as Rick pointed out, there are a ton of reasons not to worry about canon), then you'll be fine with all the HQ:G books, i e.:
Sartar: Kingdom of Heroes, The Sartar Companion, Pavis: Gateway to Adventure, The Coming Storm and The Eleven Lights. The HeroQuest: Glorantha book has a fair bit of background in it too, though a lot more rules than the aforementioned.
If you steer away from HQ completely and don't look at those, then you'll be missing out on a tremndous amount of resource (remember that HQ:G is extremely rules-lite, so the books are almost all background goodness as opposed to stat blocks and rules).