can't reach book editor
Obviously I don't know how this particular editor works (and I'm not familiar with textbook edition), but it's quite common for an academic review process to take several months or even more than a year. Another general fact about reviewing: if reviewers don't like it they don't send it to another reviewer, they write a bad review and you will be informed about it. 

Unless this editor promised you a fast review process, from the information you provide it looks to me like there's nothing to worry about. At this stage the reviewers have your book but it's possible that some of them haven't even started reading it yet. 


  I've been writing to him once a week since then--and twice last week, but with no luck.


Are you sure this editor was happy about you emailing him every week? Unless you are in very friendly terms with him, your frequent emails might be a bit annoying to him! And that might explain why he stopped answering.

Obviously I don't know any specifics, but it seems to me that there's absolutely no reason to panic: as you said, the first half was well received and you were feeling confident about it when you submitted. Try to go back to this original state of mind instead of imagining the worst. Delays are frequent in a review process and they are not a sign of negative outcome... just a sign that everybody is busy. Patience and good luck!