The Players Guide is an excellent start to get a feel for the world (and also gets your players up to speed as well).
The majority of extra supplements and adventures are based in the 2 largest continents Jaiman & Emer. There are continent overview books, region overview books and city books which each expands on the previous.
Continent OverviewFor a general view (but far more detailed info than the Master Atlas) on the continents in question, there's...
Jaiman Land of Twilight which is in the process of being updated so look out for that in the next few months.
Emer is so large, it's Continent Overview is split into 4 region books so I've listed them below.
Region OverviewHere we get a little more detailed info on these regions.
The Land of Xa-ar located in the North East of Jaiman.
The Cloudlords of Tanara located in the South East of Jaiman.
Emer I (The North West) &
Emer II (The North East) are being updated.
Emer III (The South East) which details the South East region and is available.
Emer IV (The South West) is currently in development.
City OverviewI'm sticking to cities that are within Jaiman & Emer and that have been produced recently. These describe in detail the City and it's surrounding areas.
Tales from the Green Gryphon Inn is just south of the middle of Jaiman.
Haalkitaine and the Imperial Court of Rhakhaan is to the East of the Green Gryphon Inn and is currently being updated.
Eidolon City in the Sky is located to the north of Emer. This is perhaps the most fantastical location as Eidolon is floating in the sky with Sky Ships and airships.
The
IceWebRing have a complete list of Shadow World books but only a few are currently available on RPG Now (but eBay is your friend).
Powers of Light and Darkness has overviews of the major groups and individuals of note.
There are also adventures available in the
Guild Adventurer issues.
But one of the major things in the Shadow World, whether you're playing a low powered game or if you're trying to take on the Jarak Ahrenreth are the Flow Storms and the unpredictability of their effects on players. Chapter VII of the Master Atlas has details of this as well as Essænce Barriers, Foci and Spell Manifestations (Casting spells in the Shadow World is very visual).
I also love the tech that's left behind all over the place from the First Era - some of it still works which can really put a spanner in the works of the PCs.