The textbook "Extended Chord" by Alphascript Publishing offers a comprehensive analysis and explanation of extended chords in music. It addresses the structure and function of these chords that go beyond the seventh degree, including ninth, eleventh, and thirteenth chords. The book highlights the historical development of extended harmony, starting from its rarity in the Baroque period to its more frequent use in the Classical and Romantic eras. It explains how these chords are often simplified in practice by omitting certain notes to create a specific texture. The content is based on high-quality information sourced from Wikipedia articles, providing both theoretical and practical insights into the world of extended chords.