The best beer selection in Boston will be at Sunset Grill and tap. It's on Brighton Ave, about 2 miles from fenway or so. They have a book of beer as the menu and is always changing. You really can't beat the hundreds of options for selection there. For a beer lover, it's one you can't miss.
A personal favorite of mine is Deep Ellum, just a couple blocks from Sunset. The menu isn't nearly as large, but it has some quality beers and a pretty cool atmosphere.
If you need to stay in the Fenway area, it's going to be full of bars that mostly serve your typical bar drinks. Bud light and the type. However, typical bars in Boston still always has multiple options of Harpoon, Sam Adams, and usually a few other good craft brew options. Boston is fairly spoiled in the beer world.
Enjoy your trip.