The "old masters" of classical music are always a good place to start: Vivaldi, J.S. Bach, Haydn, Handel, Mozart, Beethoven, Brahms, Tchaikovsky, etc. To expand your horizons, however, I suggest that you buy either or both of the following books, which will give you an excellent introduction to the major classical composers and the better recorded versions of their work:
1. Essential Canon of Classical Music, by David Dubal (North Point Press, 2001).
2. The NPR Guide to Building a Classical CD Collection, by Ted Libbey (Workman Publishing, 1999).
Both books are very readable and contain excellent information that will really enhance your understanding and enjoyment of the classical music repertoire.