So soon after Fiendish Codex I? Ouch, that's going to be rough because the 4e book is going to be directly compared to FCI, and that's a gigantic shadow to be standing in.
And it's really early in the 4e cycle for that sort of book, wow.
It doesn't seem early to me -- they've done undead (Open Grave) and dragons (twice! the Draconomicons). Demons seem like a good next step, especially since the next DMG seems to be focused on the epic tier -- demons should make good epic opponents.
Also, 4e's Abyss is not quite the same as 3e's Abyss, right?