I don't know which (and if) good books about Corsica are available in English, so just have a look at them at your local bookstore. As travel guide, choose the one with the most useful details and the least culture in it; it will pay you off best.
Corse Nord 73 and Corse Sud 74, 1:100,000. Published by the IGN (Institut Géographique National), called "Carte touristique IGN/France".
From the publishing house Didier Richard from the series "Itinéraires Pedestres IGN" (maps for hikers) the maps "Corse du Nord" (20) and "Corse du Sud" (23) (1:50,000); they cover about 80% of Corsica. My impression: rather bad, not very detailed, bad contour lines. They cost about 70 FF each.
"Les Cartes touristiques locales", scale 1:25,000. Much better, but also much more expensive since you'll need a lot of them (each 70 FF).
You'll hardly need a compass; the sun is shining nearly all day, and an altimeter will serve you more when hiking in the mountains. But none of these are really necessary because the routes are well-marked.