I've been an inveterate world-builder since AD&D 2e.
Here's a question to the OP: what would you consider "good support" for world-building? For me, the best support came from outside the game rules (fiction, film, real history, heck, even some poetry).
Not the OP, but, here is my take.
I find Anthropology books, History Channel, Discovery Channel, novels, movies, etc, helpful for ideas and inspiration. However, I find products that help me to, mechanically, implement those ideas and inspiration to generate new ideas, or distill the information into gaming related terms to be just as helpful or moreso. What I find useful support:
1. Rolemaster Campaign Law/GM Law
2. 2e World builder's Guide
3. TSR's 2e Green Books (e.g., Celts, Vikings, A Mighty Fortress), Expeditious Retreat's Magical Medieval Society: Western Europe (Expeditious Retreat), and GURPS Historical books (e.g, Aztecs, Celtic Myth, Japan, Vikings)
4. Green Ronin's Cavalier's Handbook, Psychic's Handbook, Shaman's Handbook, and Witch's Handbook
5. Cities 3e by Chaosium
6. Experts 3.5
7. From Stone to Steel
8. Betabunny's Bestiary: Predators, Green Ronin's Advanced Bestiary, Silverthornes' Book of Templates, Avalanche Press's Noble Steeds, and the following from Bard's and Sages: Sharks!, Snakes!, and Spiders!
9. Alchemy and Herbalists (Bastion Press/Dragonwing), Ink and Quill (Bastion Press/Dragonwing), Libem Liborium (Silven Publishing)
10.Products that help me alter or replace existing races, classes, the magic system
b. 2e PO: Spells and Magic
c. 3e Unearthed Arcana- especially, the class variants, variant class abilities, weapon groups, and alternate magic systems
d. Artificer's Handbook (ok this is magic item creation, but it redefines magic item creation)