\documentclass[letterpaper,10pt,twoside,openany]{book}

\usepackage[english]{babel}

\usepackage[utf8]{inputenc}
\usepackage{hang}
\usepackage{lipsum}
\usepackage{listings}
\usepackage{textcomp}
\usepackage{multicol}
\usepackage{dnd}

\lstset{%
  basicstyle=\ttfamily,
  language=[LaTeX]{TeX},
}

% Start document
\begin{document}

% Your content goes here

\input{features_and_traits.tex}

\newpage

\input{spells.tex}

\newpage

\input{weapons.tex}

\newpage

\input{apparel.tex}

\newpage

\input{items.tex}

% End document
\end{document}