A primary source isn't just another name for an old book. In the humanities, a document can be considered primary only if it provides direct evidence of the object under study. Thus primary sources can come in many forms such as diaries, journals, historical accounts, business records, old newspaper articles, and, yes, even books. In most cases primary sources are documents left behind by those who actually witnessed the events they describe. Working with primary sources can be very exciting because they are undigested and therefore allow you to get much closer to your subject.
The Cambridge History of the Book in BritainInterested in seventeenth-century England? Samuel Pepys's diary provides a facinating first-hand account of life in London at that time.