Gnulib is a source code library that provides basic functionality to programs and libraries. Many software packages make use of Gnulib to avoid reinventing the portability wheel. As Gnulib became such a big project over the years, we had to split this reference manual in two parts that are two separate physical books. To keep it consistent with our digital manual, the references and page numbers cover both physical books as it were one. Therefore please note that you probably want to have both parts.