Paracol: vertical line between selected columns

Question

How should I do to have vertical line only between the second and the third column? Thank you.

\documentclass{book}

\usepackage{blindtext}

\usepackage{paracol}
\setlength{\columnseprule}{1pt} 
\setcolumnwidth{0.30\textwidth, 0.30\textwidth,, 0.30\textwidth}
\setlength\columnsep{20pt}  

\begin{document}

    \begin{paracol}{3}
    \sloppy
        \switchcolumn[0]
        \blindtext[1]
        \switchcolumn[1]
        \blindtext[1]
         \switchcolumn[2]
        \blindtext[1]
    \end{paracol}

\end{document}

Answer 1

This solution uses the everypage package, assuming you want the line on every page.

\documentclass{book}

\usepackage{blindtext}

\usepackage{paracol}
\setcolumnwidth{0.30\textwidth, 0.30\textwidth,, 0.30\textwidth}
\setlength\columnsep{20pt}  

\newlength{\myoffset}
\setlength{\myoffset}{\dimexpr 0.6\textwidth + 1.5\columnsep}

\newcommand{\myhook}% cursor is at (1in,1in) and must not be moved.
{\rlap{\ifodd\value{page}\relax
    \hskip\oddsidemargin \else \hskip\evensidepargin \fi
  \hskip\myoffset\raisebox{\dimexpr-\topmargin-\headheight-\headsep-\textheight}[0pt][0pt]%
{\rule{0.5pt}{\textheight}}}}

\usepackage{everypage}
\AddEverypageHook{\myhook}% Note: you can also use \AddThispageHook, or use tikzpagenodes

\begin{document}

    \begin{paracol}{3}
    \sloppy
        \switchcolumn[0]
        \blindtext[1]
        \switchcolumn[1]
        \blindtext[1]
         \switchcolumn[2]
        \blindtext[1]
    \end{paracol}

\end{document}

---

This version uses a \tikzmark to locate the top of the paracol. One could also use (current page text area.north west) if starting at the top of the page.

Interestingly, placing the \tikzmark above the paracol doesn't work.

While both the \tikzmark and tikzpicture are invisible, they are equivalent to \hbox{} and will cause a blank line if followed by \switchcolumn.

\documentclass{book}

\usepackage{blindtext}

\usepackage{paracol}
\setcolumnwidth{0.30\textwidth, 0.30\textwidth,, 0.30\textwidth}
\setlength\columnsep{20pt}  

\usepackage{tikzpagenodes}
\usetikzlibrary{tikzmark,calc}

\begin{document}

    \begin{paracol}{3}
    \sloppy
        \switchcolumn[0]
        \noindent\tikzmark{top}\indent
        \blindtext[1]
        \switchcolumn[1]
        \blindtext[1]
        \switchcolumn[2]
        \blindtext[1]
    \end{paracol}
\noindent\begin{tikzpicture}[overlay,remember picture]
  \draw ($(pic cs:top)+(0.6\textwidth+1.5\columnsep, \ht\strutbox)$) --
    (0.6\textwidth+1.5\columnsep, \ht\strutbox);
\end{tikzpicture}

\end{document}