can't reach book editor For the last 5 years (delayed partly by teaching, a parent's death and caregiving for another parent), I've been writing a textbook that examines a famous 18th-century political work. The proposal was accepted and a year later, I got very positive feedback on the first half of the manuscript. 

I finally submitted the entire manuscript in early December 2018 and felt quite confident about it--despite the fact that my Ph.D. is in English literature and not history or poli sci.  I wrote to the editor back in late January to see if he'd gotten any word; he replied that he hadn't heard anything and to wait until late February. I've been writing to him once a week since then--and twice last week, but with no luck. It's strange because when I was corresponding with him back in August-December, he always answered very promptly. 

I've tried writing to the editor who approved the book proposal (he's since moved onto another department) and another who was a supervising editor. No response. I am really stumped: what can I do since I cannot seem to reach anyone at all? I realize there's not much action I can take and that there are any number of scenarios (maybe none of the reviewers liked it and they are sending to others?), but it is really driving me up a wall. I've looked up the "Contact us" section in the publisher's website, but there is no information on the editor of the textbook for the series that I am editing. There are no phone numbers either.  Any advice?
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!