The Maine Woods by Henry David Thoreau — Thoreau made three canoe trips through this exact region in the 1840s and 1850s, portaging rivers and climbing Katahdin, and wrote about it with characteristic precision and wonder. The landscape he describes is recognisably the same one you are moving through, which makes this essential reading before or during a trip here.
A Walk in the Woods by Bill Bryson — Bryson's account of hiking the Appalachian Trail covers overlapping territory in the Maine and New England backcountry. Funny, self-deprecating, and genuinely informative about what it takes to move through this kind of wilderness on foot.
Hiking Maine by Carey Kish — a practical guide to the state's best trails, including coverage of Borestone Mountain, Baxter State Park, and the trails around Piscataquis County. Good for planning specific hikes in the area.
The Fly Fisher's Guide to New England — covers trout and bass fishing across Maine's rivers and lakes, including the Piscataquis and surrounding waters. Useful if fishing is your primary reason for coming to this corner of the state.