oh the latex is 100% unecessary and overengineered because i get lost in it sometimes, having page headers on A5 is not even a good design choice...

anyway:

\documentclass[
a5paper,
twoside,
headings=normal,
headsepline=true,
headinclude=true,
DIV=14,
headings=optiontotoc,
numbers=noendperiod,
toc=numberline,
%draft
]{scrbook}

\usepackage[shortcuts]{extdash}
\usepackage{enumitem}
\usepackage{microtype}
\usepackage{quoting}
\usepackage[skins]{tcolorbox}
\usepackage{graphicx}
%\usepackage[homeoarchy]{impnattypo}

\usepackage[ngerman]{babel}
\babelprovide[hyphenrules=ngerman-x-latest]{ngerman}
\hyphenation{Ide-en}

% First page will be glued onto the cover
\AtBeginDocument{
  \shipout\null
  \shipout\null
}

\AtEndDocument{
  \cleardoubleoddemptypage
  \shipout\null
  \shipout\null
}

\newcommand*\rfrac[2]{{}^{#1}\!/_{#2}}%running fraction with slash - requires math mode.

% solve overfull hboxes in ToC cause by long page numbers in boldface
\RedeclareSectionCommand[tocpagenumberwidth=3em]{chapter}

% make it look like the title page
\RedeclareSectionCommand[
afterskip=0pt,
beforeskip=7em,
innerskip=0pt,
tocpagenumberwidth=3em
]{part}
\setkomafont{part}{\usekomafont{title}\huge}
\renewcommand*{\partpagestyle}{empty}

\DeclareNewSectionCommand[
counterwithin=chapter,
level=\sectionnumdepth,
style=section,
beforeskip=3.25ex plus 1ex minus .4ex,
afterskip=-1em,
indent=0pt,
font=\usekomafont{paragraph},
tocstyle=section,
tocindent=1.5em,
tocnumwidth=2.3em,
runin=bysign
]{alphsection}
\renewcommand*{\thealphsection}{\alph{alphsection})}

% need to add section title manually and make sure it's added *after* the title is set
\newcommand*{\setsectiontitle}[2]{\markright{\alphsectionmarkformat #1}}
\newcommand*{\alphsectionheading}[1]{
  \alphsection{#1}
  \AddtoOneTimeDoHook{heading/endgroup/alphsection}{\setsectiontitle{#1}}
}

\newcommand*{\thebooktitle}{}
\newcommand*{\thebooksubtitle}{}
\newcommand*{\thebookauthor}{}
\newcommand*{\thebookdate}{}

\newcommand*{\booktitle}[1]{\renewcommand*{\thebooktitle}{#1}}
\newcommand*{\booksubtitle}[1]{\renewcommand*{\thebooksubtitle}{#1}}
\newcommand*{\bookauthor}[1]{\renewcommand*{\thebookauthor}{#1}}
\newcommand*{\bookdate}[1]{\renewcommand*{\thebookdate}{#1}}

\newcommand*{\addbook}{%
  \cleardoubleevenemptypage%
  \setpartpreamble{%
    \begin{center}%
      {\usekomafont{subtitle}\thebooksubtitle\par}
      \vskip 2em
      {\usekomafont{author} {%
          \lineskip 0.75em
          \begin{tabular}[t]{c}
            \thebookauthor
          \end{tabular}\par
        }%
      }%
      \vskip 1.5em
      {\usekomafont{date}{\thebookdate\par}}
    \end{center}\par
  }%
  \part[nonumber=true,tocentry=\thebooktitle \: {\normalfont\itshape\large \thebookauthor}]{\thebooktitle}
  \setcounter{chapter}{0}
  \makeheadings{}
}

\newcommand*{\bookchapter}[1]{
  \chapter{#1}
  \makeheadings{\chaptermarkformat #1}
}
\newcommand*{\addbookchap}[1]{
  \addchap[nonumber=true]{#1}
  \makeheadings{#1}
}

\newcommand*{\makeheadings}[1]{
  \markboth{\MakeMarkcase{\thebooktitle}}{#1}
}
\begin{document}
{\huge Einführung in den Marxismus-Leninismus}

\vspace{2cm}
  \begin{tcolorbox}[
      width=10cm,
      colback=black!0!white,
      height=4cm,
      tcbox width=minimum center,
      halign=flush center,
      sharp corners=all,
      valign=center,
      enhanced,
      borderline={0.5pt}{0pt}{black},
      borderline={2pt}{-3.0pt}{black}
    ]
    {\usekomafont{title}\Huge
      Einführung \\
      in den \\
      Marxismus\-/Leninismus
      \par
      }
  \end{tcolorbox}
  \bigskip
  \vspace{2cm}
  \begin{tcolorbox}[
      colback=black!0!white,
      tcbox width=minimum center,
      halign=flush left,
      sharp corners=all,
      valign=center,
      height=6cm,
      enhanced,
      borderline={0.5pt}{0pt}{black}
    ]
    {\usekomafont{title}\Large Lohnarbeit und Kapital} - {\usekomafont{author}K. Marx} \\ \vspace{1em}
    {\usekomafont{title}\Large Dialektischer und Historischer Materialismus} - {\usekomafont{author}J. Stalin} \\ \vspace{1em}
    {\usekomafont{title}\Large Die Entwicklung des Sozialismus von der Utopie zum Wissenschaft} - {\usekomafont{author}F. Engels}\\ \vspace{1em}
    {\usekomafont{title}\Large Staat und Revolution} - {\usekomafont{author}V. Lenin} \par
  \end{tcolorbox}

\begin{titlepage}
\hspace{0pt}
\vfill
\begin{flushright}
{\huge Einführung in den \\ Marxismus-Leninismus}
\end{flushright}
\vfill
\end{titlepage}

\begin{titlepage}
    \hspace{0pt}\vfil
    \begin{center}
    \noindent
    {\usekomafont{title}\Huge
      Einführung \\
      in den \\
      Marxismus-Leninismus
      \par
      \vfill
      \vfill
      \includegraphics[height=3\baselineskip]{authors}
    }
  \end{center}
    \vfill
\end{titlepage}

\tableofcontents

\booktitle{Lohnarbeit und Kapital}
\bookauthor{Karl Marx}
\bookdate{1891}
\booksubtitle{Nachbearbeitet von Friedrich Engels}
\addbook
\include{marx2.tex}

\booktitle{Dialektischer und Historischer Materialismus}
\bookauthor{J. W. Stalin}
\bookdate{1938}
\booksubtitle{Aus "`Geschichte der KPdSU (B): Kurzer Lehrgang"'}
\addbook
%\include{stalin.tex}

\let\oldthechapter\thechapter
\renewcommand*{\thechapter}{\Roman{chapter}.}
\booktitle{Die Entwicklung des Sozialismus von der Utopie zur Wissenschaft}
\bookauthor{Friedrich Engels}
\bookdate{1891}
\booksubtitle{Auszüge aus "Herrn E. Dührings Umwälzung der Wissenschaft".}
\addbook
%\include{Engels - Die Entwicklung des Sozialismus von der Utopie zur Wissenschaft}
\renewcommand*{\thechapter}{\oldthechapter}

\booktitle{Staat und Revolution}
\booksubtitle{Die Lehre des Marxismus vom Staat und die Aufgaben des Proletariats in der Revolution}
\bookauthor{V. I. Lenin}
\bookdate{1919}
\addbook
%\include{lenin2.tex}
\end{document}

what overcomplicates this was that I put multiple books into one, so... the books themselves didn't have much styling though because I wanted to solve it all in the main.tex file so as to not hunt through all 4 books whenever I made a change