Lends credibilty and support to your research and recommendations
Allows others to reproduce your research process
Acknowledges others' contributions
Be as specific as you can: cite specific pages if possible; cite a particular chapter instead of the whole book, especially if chapter are written by different authors, etc.
There are thousands of different styles available for you to cite your sources. So while you can choose practically any style that is appropriate, it is important to stick to the style and remain consistent.
To help you create citations, you can use any of these free resources EasyBib or Noodlebib.
Details of two styles you might use are found at the Purdue Online Writing Lab (OWL) website. You will need to scroll to the bottom of the page and chose the Reference List options to see how to site a book, journal article, web site, etc.