Is having a chapter in an open-access book valuable as a publication?

For the kind of book you seem to describe, I think citations would be to chapters of the book, hence to the authors.

But pretty much any reviewed and edited publication you have helps you, though some more than others. Since you have done the work, consider the relative merits of having this vs not having it, or someone else having a chapter in your place.

If you still had the work to do, you might consider where your time and efforts would be better spent, but with the work behind you, the writing effort should pay off over time. If no one else cites it, you certainly can in work that builds on what you did. It gives you visibility with the other authors as well, and can lead to future opportunities.

I don't see any negatives other than the time spent in writing. And, saying no to the editors might not be the wisest move.

Note: I'm assuming that OP meant an open-access book (the term "open source" applies to software).

Doesn't this mean that irrespective of the chapter's content, at the end if one cites the book, only the editors of the book are cited and not me?

No it doesn't: it's very likely that your independent chapter can be cited with you as author, that's not an issue (example of an open-access book with chapters by different authors).

I am just not sure how does it help when one considers writing chapter(s) in a book helpful in academia?

In this case the open-access book is paid by your consortium, so it's not as prestigious as having a paper accepted in a competitive peer-reviewed journal for instance. But it's still a valuable publication, the editors are supposed to make sure that the chapters satisfy academic standards.

This kind of publication is more and more common due to open-access requirements by some funding bodies, for instance the EU. The advantages of this kind of publication are:

• the thematic unity of the book. Among the other chapters, your work is put in context in a way which is supposed to show its relevance.
• the freedom you have about the content. You have room to give details and, unless the editors impose specific constraints, you can present any aspect of your work. This can be an opportunity to offer a perspective that wouldn't be easily accepted in a different context.