How to cite section of a paper or chapter a book

For section of a book you can add reference of type @inbook to bibtex, but for my purposes it is usually easier to cite using

\cite[section~1.1]{foo}

 this way you can for instance cite several parts / sections / chapters from the same book or paper.

How to use multiple .bib files

Bibtex files are countaining references. In some cases we would like to use multiple .bib files, these we can simple enumerate in the  \bibliography command
as

\bibliography{file1,file2}

where file1.bib and file2.bib contain the references