\documentclass[11pt,letterpaper,openany]{scrbook}
\usepackage[letterpaper, bindingoffset=0.2in, left=1in,right=1in,top=.5in,bottom=.5in,footskip=.25in,marginparwidth=5em]{geometry}

\usepackage{marginnote}
\usepackage{sectsty}
\usepackage{ragged2e}
\usepackage{paracol}
\usepackage{fontspec}

\allsectionsfont{\centering}

\setlength\parindent{0pt}
\setlength{\columnsep}{1.25em}
\setlength{\parfillskip}{0pt}
\setlength{\tabcolsep}{1em}
\raggedbottom

\begin{document}
Quisque at nisi magna. Duis nec \textbf{lacus arcu. Morbi vel} fermentum leo. \textit{Pellentesque hendrerit sagittis vulputate.} \textbf{Fusce laoreet malesuada \textit{odio}} \textbf{, sit amet fringilla lectus ultrices porta} . Aliquam feugiat finibus turpis id malesuada. Suspendisse hendrerit eros sit amet tempor pulvinar. Duis velit mauris, facilisis ut tincidunt sed, pharetra eu libero. Aenean lobortis tincidunt nisi. \textbf{Praesent metus lacus, tristique} sed \textbf{porta non} , tempus id quam.
\end{document}