Time is now April 2026. For my own benefit, here is where the option is now, in PTXprint 3.0.18:
> Advanced > "Other" tab > Page Layout and Spacing > Where to start a new book
And a bonus-note:
You might expect to find Where to start a new chapter here:
> Advanced > "Other" tab > Chapter and Verse
yet it is not there, but rather here:
> Body > Beginning and Ending Material > Start 1st Chapter on a New Page
hth