It depends on the language (especially if you are learning not just a new spoken language, but a new system of writing), but you may want to search for “reading for X language” type books. Those can be helpful for developing reading comprehension skills (as ideally one wants to be able to read without actually translating) and typically reenforce grammar and vocabulary. Additionally, assuming you have enough of a grasp on grammar and just need to build vocabulary, then reading the news online with a dictionary can be helpful (and cheaper than buying a book dedicated to reading fluency— though more challenging).

While your circumstances may very well differ, I found conversational fluency to be extremely valuable in conducting research— whether communicating with other researchers, archival staff, or just buying food and not getting lost.

Are you asking for general advice or resources? My background/passion/obsession is in historical linguistics, so I can point you to more resources than you probably care for. I just need to know what languages are we talking about.

Since you talked about historical documents, I'm assuming you don't mean modern languages (though they can be useful, and in some cases absolutely necessary).
Will you be studying ancient languages like Latin, Ancient Greek or Old Persian? Past stages of familiar languages like Middle English or Old French? Something else entirely?

In short, what's the area you will be focusing on?