There are also a couple of new Fomorians (Dark Master and Dark Initiate) in King of the Trollhaunt Warrens, but not very much additional lore.
From earlier editions of D&D, sources for Fomorians are the 3e Monster Manual II, the 2e Monstrous Compendium Volume Two and Monstrous Manual, and the 1e Monster Manual II and, of course, Lost Caverns of Tsojcanth (S4) where they first appeared. Unfortunately, none of those have much lore, per se. Hopefully we'll get an Ecology article on them before too long, given their increased prominence in 4e.