Trio Bescherelle: The Art of Conjugation, Spelling for Everyone, Grammar for All - 3 Book Box Set (French Language Edition) | Perfect for French Learners, Students & Language Enthusiasts

This is an essential reference. Note: If one is a smart phone user the conjugation function is more conveniently and more inexpensively served by "le Conjugeur", Brewalan Le Dru, Google Play Store, free/paid, or the iOS equivalent.