I know how to use the column shortcode and create columns. I am wondering if there is a better way to enforce a column break than manually adding blank lines at the end of the the first column?
I am quite comfortable with HTML and CSS if anyone has suggestions that involve modifying custom styles or adding code in the text editor.
I am not sure wether you are referring to the print version or online version of your book.
Here are some remarks about columns and balancing columns in general (assuming you can add custom css in the layout section of Pressbooks)
For the print version the below might help, assuming you are using princexml for pdf creation (the hosted version of Pressbooks does use princexml via docraptor service):