If it is copied from a public source, copy the complete title of the book, year of publication, name and address of the author, and page number. This will help you or someone else locate the information again. What ever method you choose to use, be consistent. Keep all correspondence for further reference. Letters contain addresses and dates which may be helpful in the future.
Old letters are invaluable.
Not only do they tell you where a family member was at a particular time, they also give you little bits of family history that may be vital in the search for a missing relative. A file cabinet is always handy for organizing this correspondence. Always recognize any copyrighted material you may come across. The author usually has worked hard compiling the material and deserves recognition. Please request written permission from the author before using any copyrighted material for any reason.