[Serpentine]

Ah, it looks like some of the CSS in that is causing Calibre to replace all of the spaces with non-breaking spaces. As such it just sees entire paragraphs as a single long word, when it does line wrapping it cant try to cut at white spaces. i.e.,

<p class="calibre18"><span class="calibre7">“Never&nbsp;imagined&nbsp;I’d&nbsp;be&nbsp;a&nbsp;babysitter,”&nbsp;I&nbsp;hear&nbsp;SNIPno&nbsp;idea&nbsp;I’m&nbsp;grooming&nbsp;the&nbsp;horse&nbsp;in&nbsp;the&nbsp;next&nbsp;stall.</span></p>

You can fix this by removing: white-spacere-wrap
From the styles, in specifically the divs that wrap each chapter.