\documentclass[a5paper, 12pt]{book}
\usepackage[paperwidth=15.4cm, paperheight=21.59cm]{geometry}
%\usepackage[paperwidth=12.4cm, paperheight=17.3cm]{geometry} %für Ebook
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\pagestyle{plain}
\usepackage{graphicx}
\usepackage[defaultlines=2,all]{nowidow}

\begin{document}
\input {TXT/titelei.txt}
...