Where was John Bolton during the Trump impeachment hearings?

I haven't read John Bolton's new tell-all book, and I'm pretty sure I never will (not my idea of enjoyable bedtime reading).
But, with all the revelations that have already been leaked to the press from it, my question would be: where was Bolton during the Trump impeachment hearings? The answer presumably is: writing this book. This kind of testimony could well have swung the decision the other way, and yet Bolton refused to testify. I guess that might have impacted on his book sales...
All we can hope now is that Americans read it before the election date. The Trump administration's attempts to block the book's publication "on national security grounds" (AKA censorship) will probably help sales roll along nicely.