The first edition of the OOo3 Impress Guide is now available. Individual chapters and a compiled book are on the OOoAuthors website (ODT and PDF), the Documentation Project website (PDF), and the Documentation wiki.
Note: This first edition really could use some more reviewing, editing, and indexing. The choice was between publishing a (hopefully not seriously) flawed and incomplete book now, for people to use and comment on, or wait indefinitely for enough assistance to do a better job. I chose to make the info available.
I don’t plan to work on a printed copy any time soon, because (a) I have higher-priority things to do, (b) the OOo2 Impress Guide hasn’t sold many copies (in contrast to the Getting Started and Writer Guides for both OOo2 and OOo3), and (c) it still needs work.