The Allen Forte Archive @ University of North Texas 
Digital documents are available for download there. Please note that we have the actual Forte Papers here at Yale

Animated Schenkerian anlaysis of Schumann's Träumerei, op. 15/7 & English horn solo in Tristan's 3d act
These are flash-based rendering of the pieces' analyses.

Luciane Beduschi & Nicolas Meeùs -- Théorie et analyses schenkériennes 
An abundant number of analyses are provided by the two scholars including original analyses by Schenker (free PDFs.)

Oxford Music Online 
The Grove Music Dictionary Online has a great article on Schenker (of course!) and an extensive bibliography. This can be a great starting point to find authoritative articles about your topic. http://www.oxfordmusiconline.com/subscriber/article/grove/music/24804

 Schenker Documents Online  
This contains a pretty extensive list of documents relating to Schenker or written by and/or to Schenker. There are two websites with the same name (Schenker Documents Online). It is unclear if this second one duplicates the content of the first one.

Cambridge History of Western Music

Chapter 26 deals with Schenker. Available through our e-books selection (via Orbis)

Neue musikalische Theorien und Phantasien: Harmonielehre v. 1

Pamphlets in witness of the immutable laws of music. Schenker, Heinrich, 1868-1935.
To access this online book you must be a Yale affiliate and logon via VPN is off-campus.

Salzer, Felix. Structural hearing [electronic resource] : tonal coherence in music

 

 Snarrenberg, Robert. Schenker's interpretive practice