When you are reading a book, a book chapter, journal article, document on the web etc., it is important that you note down the bibliographic details (author, title year of publication, URL etc.,) of the material. This is because you will have to cite the resources you used in your work. Failing to cite or reference the resource you used to complete your assignment or project work, is a breach of copyright law and you can be charged for plagiarism. Remember to always cite your sources of information at the end of your work.
The following books can link you to different styles of referencing or citation.