ORY is closer to central Paris than CDG. One easy routing is ORLYVAL to Antony station (last stop on that line) on the RER B, and then a few stops north to St. Michel, would get you very close to the 93 Blvd. St. Michel address mentioned. The others are not too close to that one. If you have a lot of luggage, the route above would NOT be very convenient. A taxi might be the most efficient routing.
A Carte Orange (weekly or even for the month, depending on the date of arrival) would be very useful in navigating transportation in Paris.
If you go to
www.ratp.fr (and select English) you can easily navigate that sight and even place departure and arrival points and get a full itinerary of the optimal routing to your destination. There are hundreds of guide books about Paris. Some are oriented for people who are students--go to any large bookstore and browse the shelves.
