If you're doing research on 18th century India, chances are good you've run across the names of one or two Jesuit priests. They appear in the landscape of southern India with especial frequency, but also toward the north/central region (there was a Jesuit church and college at Agra, for instance). An incredibly curious set of religious folk, the Jesuits took notes on just about everything they saw: language, religion, family, geography, science, trade, travel, etc. We know they were curious, because they wrote letters with unbelievable frequency when they were out on mission, describing the world as they encountered it.
Fortunately, many of the letters written back to Europe from India, China, the Americas and Africa were preserved by the recipient (usually a priest in a higher position). In the eighteenth century, 34 volumes worth of letters were edited and published by the Society of Jesuits under the title Lettres édifiantes et curieuses (you can read a brief summary of the series at Wikipedia, of all places). It takes time to sort out the various editions and publication dates for the volumes, especially if you're trying to do it through interlibrary loan, but I think it's worth it. I've been using the 1781 edition (occasionally reading the 1819 edition for newer punctuation and spellings) because that takes me well beyond the end date for my project (1743).
Unfortunately, even with 34 volumes, the collection doesn't include every letter ever written by every Jesuit priest in the world. That's where the Jesuit Archives in Vanves (Paris) come in handy. I've just started working here, and can I say, this is a really great research experience. Aside from my inability to use the doorbell properly, it's been very painless, even with my abominably poor French (I read it much better than I speak it, obviously). Le Père Archiviste, Robert Bonfils, is friendly and helpful. He speaks perfect English and didn't make me feel stupid for lacking the same fluency with his own language.
Today I looked at two volumes of the Fonds Brotiers, reading letters from various Jesuits interested in astronomy in India and China. These letters nicely fill in the gaps left in the Lettres édifiantes. I think I took the right approach--spent quite a bit of time with the published letters, worked out a timeline and familiarized myself with names and personalities, and then went to the archive. The names I'm reading are all familar to me, and I've read published letters by all of them. This has been surprisingly helpful when I'm trying to decipher complex handwriting.
Practicalities: I had a native French speaker translate my letter of introduction into French for me, but I'm not sure that is necessary. At any rate, you can contact the archivist at:
Archives de la Province de France de la Compagnie de Jésus
15 rue Raymond Marcheron
Contacting Mr. Bonfils ahead of time helped me discover that a citation reproduced a hundred times over in the secondary literature on my subject was incorrect.
The archive opens at 9 a.m., and is closed for lunch from 12:00-1:30, which will give you plenty of time for a nice lunch.
Unlike the British Library, this archive permits the use of a camera, useful for taking images of fragile documents.
There are two Citéa hotels within a two-minute walk of the archive. I can say that Citéa Vanves Porte de Châtillon is clean and comfortable (it looks exactly as the website promises), and it takes me 45 seconds to walk from the front door of the hotel to gate of the archive, 4-5 minutes to walk to the grocery store, and 9-10 minutes to walk to the metro. So, convenient, and inexpensive at Euro65/night.