Music Theory in Mamluk Cairo by 
A Theory of Musical Narrative by
Byron Almén proposes an original synthesis of approaches to musical narrative from literary criticism, semiotics, historiography, musicology, and music theory, resulting in a significant critical reorientation of the field. This volume includes an extensive survey of traditional approaches to musical narrative illustrated by a wide variety of musical examples that highlight the range and applicability of the theoretical apparatus. Almén provides a careful delineation of the essential elements and preconditions of musical narrative organization, an eclectic analytical model applicable to a wide range of musical styles and repertoires, a classification scheme of narrative types and subtypes reflecting conceptually distinct narrative strategies, a wide array of interpretive categories, and a sensitivity to the dependence of narrative interpretation on the cultural milieu of the work, its various audiences, and the analyst. A Theory of Musical Narrative provides both an excellent introduction to an increasingly important conceptual domain and a complex reassessment of its possibilities and characteristics.
Music theory reference, tutorials, exercises, and articles. This site is run by a professor at the Conservatory of Music of Puerto Rico; the site is available in both English and Spanish.
A free online course on the fundamentals of music theory
Ricci Adams's MusicTheory.net has a number of free tools for learning basic music theory, including customizable ear training sessions, elementary notation, rhythm and meter, and key signatures. Matrix calculators (12-tone) and a staff paper generator are also included. A very handy website for the music theory student.
Musicologist Peter Schubert has uploaded videos discussing and demonstrating live improvising and improvisation of a canon.
Peter Schubert and Julie Cumming's video series on Renaissance improvisation practice
Online flashcards for music theory. Includes flashcards for reading the tenor and alto clefs.
Online exercises and learning tools in foundational music theory, music notation, and ear training.
While not precisely an encyclopedia, this is a very rich source of information for the basics of notation, meter and rhythm, chord structures, and so on. The Dolmetsch Foundation is based in England, and so their dictionary is helpful for understanding English or European notational and vocabulary conventions that are different from American music theory conventions.
A collection of links that cover the basics of reading common-practice musical notation.
