Great choice Dawn - the International book store is 3 stories high. On the top floor they have a LARGE English book section at pretty reasonable prices. This is a GREAT place to pick up your Guides to Rome... and other parts of Italy! The bookstore is in the front right quadrant of Termini Station if you're looking from face-on outside the building.
Termini also has a large grocery store in the bottom level.. and bottled water was just 26 cents (Euros) last week - WOW!!! Termini may not be the prettiest train station in Europe - okay it's ugly and often dirty - but it is effective and does have some reasonably priced shopping... Unfortunately the bathrooms are still 70 cents!
Obviously I've spent TOO MUCH time there waiting on friends and family..... Ciao for NOW,
Ron