Researchers?  We are all researchers, so we know best how to write to ourselves as an audience.  But that might limit interest in the book.

Practitioners?  That means programmers and a broader audience than researchers.  If we are aiming for practitioners, should we cut user study information from our chapters?

End users?  Terminology problem: no one wants to self-identify as an end user.  End users would also want to download and use a system immediately, but not all chapters describe available systems.

Suggestion for publisher: web community in parallel with book.  Publishers get excited about this.  Many of the chapters are community-oriented anyway.