I have discovered that a co-author has submitted a book chapter to a major publisher omitting my name of co-author list without me knowing. I was included in the first submissions, but corresponding author made an executive decision to not include me, based on unknown grounds. I have substantially and creatively contributed to the work, in design, datacollection and analysis. What are my options now to get credit where credit is due? technically it is plagiarism, I think.
It may or may not technically be plagiarism, but it’s definitely fraud if it was done without your consent. Papers and book chapters can be retracted for such infractions.
If this has indeed happened, then your next step—assuming you've reached an impasse with your co-authors—should be to contact the editor and publisher of the book, laying out your case with the appropriate documentary evidence included in its entirety. Make sure they have enough information that they’re not going on a “fishing expedition.” There should be clear correspondence—emails showing the work and production of the article with your participation, and so on.
It's easy for people to make stupid mistakes, especially if they had to write the author list in a new form. (Harder to accidentally delete one name from a pre-existing list, though.) I recently got an email from coauthors saying, "Oops, we forgot to put your name on the most recent submission," after being on a prior conference submission. They promised to make it right on the next phase of the project.
The absolutely first thing to do is to approach the corresponding author directly about this mistake, and politely ask them to correct it in the current draft (or with the editor and publisher, if it's that far along). They will either do so or give you more information. Even if it was somehow intentional, treating it like a mistake allows them to change course but still save face.
Before you approach them about this, be prepared to escalate the issue as aeismail suggests. If they refuse to add you back on, then contact the editor and publisher. You can also add, "The corresponding author claimed X when I asked about this today, but clearly the emails enclosed show !X." That is, if the other person is acting in bad faith, then they might try to talk with the editor and publisher before you do; and there is some benefit to being able to tell the story of a conflict first.