You have just learned how to include the details of a book in a reference list. You now need to complete your reference by citing the author in your text. In the Harvard system this means including the author's surname and the date of the work. The citation is included at the point at which you are referring to the book.
If you wish to include the author's name within a sentence then you use the form:
Evans (1992) offers plenty of useful advice for young cat owners...
Note that only the date is in parentheses.
If you don't want to use the author's name directly then you would write:
Although some authors are concerned mainly with how people can provide for their cats (Evans, 1992; Pope, 1986), others observe that the relationship between cat and man can be mutually beneficial (Clutton-Brock, 1981).
Here, both the surname and the date are within brackets, and separated by a comma. Where more than one work is cited the details are punctuated by semi-colons.
As with the reference list it is important that you are consistent with the style of your citations.