It can vary a bit from river system to river system, with some patterns seeming to be more effective, and to some extent when you go- some rivers get a run of Arctic Char fresh from the sea in August, and many areas may have pike and landlocked Arctic Char as well as Brook trout, so you may want to consider some patterns for them too if the spot you're going to has them.
Bring a box of dries, say 16 and up, but expect to spend most of your time throwing muddlers, streamers, bucktails and larger dries like Bombers and large Wulffs as well as deerhair mice (!!!). The brookies mean business up there, and can put on a 1lb over the summer, so they are looking for some big happy meals, so think BIG.
Again, speak to your outfitter if you've booked a trip for specifics on the waters you plan to fish for the time you'll be there, but a good place to start would be the Minipi Camps website for patterns to start tying.
Their website has a brief snap shot of the flies and sizes that are the most successful here:
The Minipi Camps Fly Box
and a more detailed listing in a .pdf file on their site, as well as info on gear to bring:
Good luck, it sounds like a great trip.