updated preface

Author: thomasWeise

text/front/preface/preface.tex

@@ -15,12 +15,14 @@ \chapter*{Preface}%
 This book is work in progress.
 It will take years to be completed and I plan to keep improving and extending it for quite some time.
 
-This book is freely available.
-You can download its newest version from~\citeurl{programmingWithPython}.
-This version may change since this book is, well, work in progress.
-The book is released under the \emph{Attribution-NonCommercial-ShareAlike~4.0 International license} (\href{http://creativecommons.org/licenses/by-nc-sa/4.0/}{\mbox{CC~BY-NC-SA~4.0}}).
-You can freely share it.
-You can also copy text or figures, as long as you cite the book as the original source~\expandafter\url{\programmingWithPythonUrl}, e.g., by using the following Bib\TeX:%
+The book consists of two types of material:
+Materials that the author~(Thomas Weise) has created by himself and such that have been created by others.
+The vast majority of the material is teaching material created by the author.
+This and only this material is released under the \emph{Attribution-NonCommercial-ShareAlike~4.0 International license}~(\href{http://creativecommons.org/licenses/by-nc-sa/4.0}{\mbox{CC~BY-NC-SA~4.0}}).
+However, the book also includes some images and figures created by others, which are marked explicitly and licensed under their authors' terms.
+For example, all logos and trademarks are under the copyright of their respective owners.
+
+You can cite this book~\cite{databases}, e.g., by using the following Bib\TeX:%
 %
 \begin{lstlisting}[style=text_style]
 @book{programmingWithPython,