Oh your hint with a minimal example finally solved the problem and I could figure it out on my own. Sorry for this, I will keep the minimal example in mind
I'm using the thebibliography environment to create a list of all cited book. One thing I noticed is that whenever I create a new section and cite a book, there will be the section number in the bibliography at the end of the cited book. For example: \subsection{section5}\label{sec:section5 ...