D&D 4E Tell me about these "preview books" for 4e

[TerraDave]

Honestly, I very much doubt they will be all fluff. These books have likely already gone to press. My bet is, whatever major changes they are making to the races/classes and world/monsters are finalized already.

EDIT: This is a big tip off for me about playtesting. I believe it will mostly be shaving off any unforeseen bumps; it will not be a time to rebuild whole systems.

And you may be right...in Chris Perkins latest blog entry (post on the blog thread) he says:

In 4th Edition, all characters have a selection of at-will, per-encounter, and per-day resources. The exact mechanical execution of this base concept will be disclosed in the coming months on D&D Insider and the Wizards Presents: Races and Classes preview book, so no spoilers here!

Though I am surprise they would release books that would overlap so much with the PHB.