The Craft of Sail: A Primer of Sailing by Jan Adkins

Subtitled "a primer of sailing" it really is an excellent guide to understand how wind and water affect boats. Aside from basic seamanship, Jan also includes drawings of the primary types of sailboats, so you'll see what makes a cutter a cutter, a schooner a schooner, etc. The illustrations will hold the attention of young readers, while older readers will appreciate the clear, concise explanations of difficult concepts.