Well, while I try to wake my brain up at work. . .
3.5 Sourcebooks and Rule BooksMagic of Arcanis - Highly recommended to those who wish to play casters in the 3.5 version, or for the wealth of fluff and information for those who play the ARPG version.
The Players Guide to Arcanis - The Essential Core Book for the 3.5 version. Even in the ARPG version of the game I find myself scanning through the wealth of knowledge here.
Arcanis Superpack - Collection of the core rules books for the 3.5 version of the game.
Codex Arcanis - A HUGE resource on the history of Arcanis. A bit dated and 3.0-heavy in its tropes, but it's free, so download it!
Psionics Unbound - Like the Magic book, this book is essential for any Psionic classes in 3.5 Arcanis, as well as providing all the fluff you'd ever want on the subject (ie: What is the Silence?)
Ssethregore: In the Coils of the Serpent Empire - This book is slated for an ARPG makeover, but until then catch a glimpse of the evil Lizard Empire!
Theocracy of Canceri - Like Ssethregore, this is due for a makeover, but until then it offers lots of juicy information!
3.5 Adventures and City GuidesCity of Secrets: The Adventurer's Guide to Nishanpur - This book details the history and present of the City of Nishanpur: The Capital of the Theocracy of Canceri. Contains a short adventure as well.
Shadows of a Forsaken Past - Probably one of my favourite adventures of the 3.5 days!
Year of Ill Harvest - Year 1 of the 3.5 campaign. If you are at all interested in the old campaign, this book is where you start.
Year of the Fall - Year 2 of the 3.5 campaign, and is also essential if you want to give a crack at that version of the game.
Carnival of Swords - A "Pre-Living Arcanis" Adventure book set in Old Coryan, the original capitol of the Coryani Empire! So much background information on the Coryani it is intense!
"Everything Evil" - Collection of Nishanpur, Theocracy of Canceri, and the Blood Reign of Nishanpur (which is a "Pre-Living Arcanis" adventure book!)
Other useful 3.5 productsEldest Sons: The Essential Guide to Elves - Mostly unrelated to Arcanis, but the final chapter gives probably the single-most dense concentration of information on the Elorii, at least those of Entaris.
Arcanis, the Roleplaying GameArcanis: The Roleplaying Game - The Core Book for the ARPG gameset. Highly recommended!
Arcanis Bestiary, volume 1 - If you want to see how to make bad-guys of Arcanis and see what kind of terrifying threats there are on Onara, this book is for you!
Arcanis: The Codex of Heroes - This book effectively doubles character creation and advancement options!
Forged in Magic, Volume 1 - A collection of magical items, including the original batch of playable "Runes".
Forged in Magic, Volume 2: Treasures of the Ages - Even more magical items, including more runes and rules for Runestaves!
Knightly Order of the Phoenix - Interested in Milandir? Well, look at one of the more unique and secretive Knightly Orders!
ARPG AdventuresSo Shall Ye Reap - The first adventure of "Year of Ill Harvest", but updated to ARPG rules.
Also: Until January 28, 2014, you can get all the ARPG supplements for cheap at the KICKSTARTER!!!!