I'd say one of the Master Atlases is probably the first purchase. After that, where you are going to be playing is fairly relevant. If you have any of the old stuff for the areas, they may be useful. The two real choices are currently Jaiman or Emer.
Jaiman: Currently there is The Land of Xa-ar and Northern Saralis. Adventures from Guild Adventurer 2 and 3 are set in the same region. Tales from the Green Gryphon is set in Rhakhaan, but frankly could be placed anywhere you want. The Cloudlords of Tanara is set in the south east of Jaiman. There is also stuff in progress for that continent, but it may take a while. Quelbourne, an old supplement still available in PDF, is near Xa-ar as well.
Emer: Emer Atlas III and Eidolon, which is in a different region to the Emer III (it's in Emer II), are what's currently ready. You may also be able to pick up old copes of Emer I and II which are being revised. An adventure in Guild Adventurer 1 is set in the region of the currently unavailable Emer II and one in GA4 is set in Emer III. The free download Dagger of Jade, available from the Vault here, is also set in Emer III. Emer I may be the next new Shadow World supplement to be released, going by what Terry has posted.